I have a long-term problem with a 2017 office desktop (ASUS H110M-R, i5-7400, OS: Linux Mint 19.2, kernel 4.15.0.70) that wakes up from ACPI S5 randomly once or twice a month which is annoying.
It's powered from a UPS with a surge protection but there's a ~5m power cord between the UPS and the PC.
BIOS doesn't support wake on LAN.
There's no BIOS option to disable wake up by a keyboard press, so the impulse could possibly come also from this source but nobody presses a key.
Syslog doesn't tell what triggered the wake up.

Can anybody point me what to check and how to prevent those unwanted wake ups? Thanks.

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Nov 25 22:53:15 wd93rw71 kernel: [58740.729012] PM: suspend entry (deep)

Nov 26 00:06:51 wd93rw71 kernel: [58740.729015] PM: Syncing filesystems ... done.
Nov 26 00:06:51 wd93rw71 kernel: [58740.742250] Freezing user space processes ... (elapsed 0.001 seconds) done.
Nov 26 00:06:51 wd93rw71 kernel: [58740.743940] OOM killer disabled.
Nov 26 00:06:51 wd93rw71 kernel: [58740.743940] Freezing remaining freezable tasks ... (elapsed 0.001 seconds) done.
Nov 26 00:06:51 wd93rw71 kernel: [58740.745116] Suspending console(s) (use no_console_suspend to debug)
Nov 26 00:06:51 wd93rw71 kernel: [58740.786294] sd 0:0:0:0: [sda] Synchronizing SCSI cache
Nov 26 00:06:51 wd93rw71 kernel: [58740.786295] sd 1:0:0:0: [sdb] Synchronizing SCSI cache
Nov 26 00:06:51 wd93rw71 kernel: [58740.786472] sd 1:0:0:0: [sdb] Stopping disk
Nov 26 00:06:51 wd93rw71 kernel: [58740.786689] serial 00:02: disabled
Nov 26 00:06:51 wd93rw71 kernel: [58740.786908] parport_pc 00:01: disabled
Nov 26 00:06:51 wd93rw71 kernel: [58740.787087] sd 0:0:0:0: [sda] Stopping disk
Nov 26 00:06:51 wd93rw71 kernel: [58741.531795] ACPI: Preparing to enter system sleep state S3
Nov 26 00:06:51 wd93rw71 kernel: [58741.741013] PM: Saving platform NVS memory
Nov 26 00:06:51 wd93rw71 kernel: [58741.741112] Disabling non-boot CPUs ...
Nov 26 00:06:51 wd93rw71 kernel: [58741.756469] smpboot: CPU 1 is now offline
Nov 26 00:06:51 wd93rw71 kernel: [58741.779459] IRQ 123: no longer affine to CPU2
Nov 26 00:06:51 wd93rw71 kernel: [58741.780472] smpboot: CPU 2 is now offline
Nov 26 00:06:51 wd93rw71 kernel: [58741.799429] IRQ 122: no longer affine to CPU3
Nov 26 00:06:51 wd93rw71 kernel: [58741.800455] smpboot: CPU 3 is now offline
Nov 26 00:06:51 wd93rw71 kernel: [58741.803592] ACPI: Low-level resume complete
Nov 26 00:06:51 wd93rw71 kernel: [58741.803690] PM: Restoring platform NVS memory
Nov 26 00:06:51 wd93rw71 kernel: [58741.805823] Enabling non-boot CPUs ...
Nov 26 00:06:51 wd93rw71 kernel: [58741.805854] x86: Booting SMP configuration:
Nov 26 00:06:51 wd93rw71 kernel: [58741.805855] smpboot: Booting Node 0 Processor 1 APIC 0x2
Nov 26 00:06:51 wd93rw71 kernel: [58741.807504]  cache: parent cpu1 should not be sleeping
Nov 26 00:06:51 wd93rw71 kernel: [58741.807549] intel_pstate: Disabling energy efficiency optimization
Nov 26 00:06:51 wd93rw71 kernel: [58741.807657] CPU1 is up
Nov 26 00:06:51 wd93rw71 kernel: [58741.807686] smpboot: Booting Node 0 Processor 2 APIC 0x4
Nov 26 00:06:51 wd93rw71 kernel: [58741.809197]  cache: parent cpu2 should not be sleeping
Nov 26 00:06:51 wd93rw71 kernel: [58741.809336] CPU2 is up
Nov 26 00:06:51 wd93rw71 kernel: [58741.809360] smpboot: Booting Node 0 Processor 3 APIC 0x6
Nov 26 00:06:51 wd93rw71 kernel: [58741.810890]  cache: parent cpu3 should not be sleeping
Nov 26 00:06:51 wd93rw71 kernel: [58741.811034] CPU3 is up
Nov 26 00:06:51 wd93rw71 kernel: [58741.813438] ACPI: Waking up from system sleep state S3
Nov 26 00:06:51 wd93rw71 kernel: [58741.915611] rtlwifi: rtlwifi: wireless switch is on
Nov 26 00:06:51 wd93rw71 kernel: [58741.919527] sd 0:0:0:0: [sda] Starting disk
Nov 26 00:06:51 wd93rw71 kernel: [58741.919530] sd 1:0:0:0: [sdb] Starting disk
Nov 26 00:06:51 wd93rw71 kernel: [58741.922490] parport_pc 00:01: activated
Nov 26 00:06:51 wd93rw71 kernel: [58741.924769] serial 00:02: activated
Nov 26 00:06:51 wd93rw71 kernel: [58741.944863] rtc_cmos 00:07: Alarms can be up to one month in the future
Nov 26 00:06:51 wd93rw71 kernel: [58741.968681] nct6775: Enabling hardware monitor logical device mappings.
Nov 26 00:06:51 wd93rw71 kernel: [58741.973513] r8169 0000:03:00.0 enp3s0: link down
Nov 26 00:06:51 wd93rw71 kernel: [58742.269638] ata4: SATA link down (SStatus 4 SControl 300)
Nov 26 00:06:51 wd93rw71 kernel: [58742.269796] ata1: SATA link up 6.0 Gbps (SStatus 133 SControl 300)
Nov 26 00:06:51 wd93rw71 kernel: [58742.269846] ata3: SATA link up 1.5 Gbps (SStatus 113 SControl 300)
Nov 26 00:06:51 wd93rw71 kernel: [58742.270796] ata1.00: configured for UDMA/133
Nov 26 00:06:51 wd93rw71 kernel: [58742.286749] ata3.00: configured for UDMA/133
Nov 26 00:06:51 wd93rw71 kernel: [58743.314558] [drm] RC6 on
Nov 26 00:06:51 wd93rw71 kernel: [58743.513811] r8169 0000:03:00.0 enp3s0: link up
Nov 26 00:06:51 wd93rw71 kernel: [58744.888997] acpi LNXPOWER:16: Turning OFF
Nov 26 00:06:51 wd93rw71 kernel: [58744.889238] acpi LNXPOWER:15: Turning OFF
Nov 26 00:06:51 wd93rw71 kernel: [58744.889472] acpi LNXPOWER:14: Turning OFF
Nov 26 00:06:51 wd93rw71 kernel: [58744.889705] acpi LNXPOWER:13: Turning OFF
Nov 26 00:06:51 wd93rw71 kernel: [58744.889938] acpi LNXPOWER:12: Turning OFF
Nov 26 00:06:51 wd93rw71 kernel: [58744.890170] acpi LNXPOWER:11: Turning OFF
Nov 26 00:06:51 wd93rw71 kernel: [58744.890402] acpi LNXPOWER:10: Turning OFF
Nov 26 00:06:51 wd93rw71 kernel: [58744.890681] acpi LNXPOWER:0f: Turning OFF
Nov 26 00:06:51 wd93rw71 kernel: [58744.890915] acpi LNXPOWER:0e: Turning OFF
Nov 26 00:06:51 wd93rw71 kernel: [58744.891147] acpi LNXPOWER:0d: Turning OFF
Nov 26 00:06:51 wd93rw71 kernel: [58744.891380] acpi LNXPOWER:0c: Turning OFF
Nov 26 00:06:51 wd93rw71 kernel: [58744.891613] acpi LNXPOWER:0b: Turning OFF
Nov 26 00:06:51 wd93rw71 kernel: [58744.891845] acpi LNXPOWER:0a: Turning OFF
Nov 26 00:06:51 wd93rw71 kernel: [58744.892078] acpi LNXPOWER:09: Turning OFF
Nov 26 00:06:51 wd93rw71 kernel: [58744.892310] acpi LNXPOWER:08: Turning OFF
Nov 26 00:06:51 wd93rw71 kernel: [58744.892543] acpi LNXPOWER:07: Turning OFF
Nov 26 00:06:51 wd93rw71 kernel: [58744.892777] acpi LNXPOWER:06: Turning OFF
Nov 26 00:06:51 wd93rw71 kernel: [58744.893009] acpi LNXPOWER:05: Turning OFF
Nov 26 00:06:51 wd93rw71 kernel: [58744.893240] acpi LNXPOWER:04: Turning OFF
Nov 26 00:06:51 wd93rw71 kernel: [58744.893472] acpi LNXPOWER:03: Turning OFF
Nov 26 00:06:51 wd93rw71 kernel: [58744.893634] OOM killer enabled.
Nov 26 00:06:51 wd93rw71 kernel: [58744.893634] Restarting tasks ...
Nov 26 00:06:51 wd93rw71 kernel: [58744.893709] usb 1-6.2: USB disconnect, device number 6
Nov 26 00:06:51 wd93rw71 kernel: [58744.893780] usblp0: removed
Nov 26 00:06:51 wd93rw71 kernel: [58744.894685] done.
Nov 26 00:06:51 wd93rw71 kernel: [58744.962996] PM: suspend exit
...

Keyboards/mice sometimes react to interference like switch on of lighting in the same room (the electrical interference, before the lamp makes its first flash, not optical vs mouse or anything like that). Try to unplug the cables from the PC once it is sleeping and see which one makes a difference

Some particularly bad power supplies can cause this - If +5Vs goes out of regulation on the low side or have excessive ripple, sometimes this will show up also on the PS ON signal, and might make the PS switch on by itself. Most PCs will spin the fans etc but not otherwise "wake up" just by applying full power to them without also intentional actions from bios/software, but maybe the applied power could in turn make such actions

Some particularly bad power supplies can cause this - If +5Vs goes out of regulation on the low side or have excessive ripple, sometimes this will show up also on the PS ON signal, and might make the PS switch on by itself. Most PCs will spin the fans etc but not otherwise "wake up" just by applying full power to them without also intentional actions from bios/software, but maybe the applied power could in turn make such actions

I've had that happen to me before with dodgy externally powered USB hubs. Sometimes it would affect the 5v rail enough to trigger the motherboard to boot up.