It was, although I never saw the panel in action, only in idle state through the window in the door. I did have a conversation about its workings with the head of maintenance, though. I knew the guy aside from the university. There is a slight chance the panel now belongs to a local firefighter museum, but I'm not sure of that as it was replaced after he left his job. If ever I get to see it I will post pictures.

It wouldn't be hard to use this principle in other applications, the operation is quite simple.

For the ionization detector workings and low battery warning activating just at night:
That is how the main detection works, but the question was, how is implemented the low batt warning - if it's activation just at nights was just a coincidence or if there is some mechanism related to that.
There could be constructed some "chain of events" to explain such dependency, but it does look too complex and with too many assumptions to be plausible. So I guess it was more of a coincidence (probably some have tripped the LowBatt during the day as well, but as there was nobody in the house, no one registered it before the battery have died completely)

Maybe lower temperatures at night impede the performance of the battery, thus tripping the low battery alarm.

That could be an explanation, indeed...

At my house I got a 1970's mid 80's burglar alarm system called glo-bell alarms it has the telephone number on the bell box and a large metal panel the P.I.R's on the system are racal guardall Apollo d's and a rare aritech pir I think it is I'm not really sure but one of the pirs are a guardall Scorpio witch is dead. In the hallway we had a master blaster siren and you still see them at some places in the UK

Found a pair of old General Electric alarms at the ReStore in Birmingham, AL today.  These are almost identical to the ones I remember when I was little, except those were 9 volt and these are 120v hard wired.  Apparently they were also sold as hard wired with battery, 120v plug-in and a version that used an unusual 12.5 volt battery.

Not usually interested in smoke alarms, but i have this one i found in an RV at the junkyard, not sure if it works, never caught anything on fire to test it   but it appeared to be old so i snagged it, it still chirps when i hit the test button...

it needs a new battery clip though... the only one was absolutely destroyed so i snipped it

I'm not greatly interested in alarms though i do mind them and notice them

Got an old (made in 1994, but the design looks like it dates way back) burglar alarm panel (main box) some time ago from a building renovation (along with couple 100's Fluorescent lanterns). The thing appears to be very basic. No keypad or anything like that. The arm/disarm is a mechanical key lock, and the status display is an array of 5mm LED indicators. And i got the siren from it as well

Being simple relay contact control, i could replace the key lock to a new one (there is no key to the original) and use it with modern relay contact PIR / window sensors

I guess i could use it as a fire alarm too if i find smoke detectors with relay contact outputs (those appear to be uncommon, atleast nowadays, its either proprietary digital stuff or stand alone ones with a battery)

at my house we used to have an 9 volt GE Home Sentry Smoke alarm
and whenever it went off it always hurt my ears