1) check for a battery back-up Carbon Monoxide alarm - might be signalling a low battery.

2) Might be a frisky CRICKET (literally an insect looking to attract a mating companion).

Just in case it is a smoke detector or Carbon Monoxide detector either battery powered or battery back-up signalling a low battery (some older CO detectors first chirp at lowlevel detections below the "emergent" threshold from the upstairs or common wall neighbor - Make a visit to either/or and see if they might have an chirping alarm. If either has been vacant for a while contact your building maintenance/superintendant or property manager and/or the Condo Board and express your concern.

Do you have a battery back-up device (like for a computer, a life-safety alarm system - like "i've fallen and can't get up system, or similar in the condo? These tend to chirp whenever they are tripped into the ON position and/or if they are unplugged but not turned off.

A personal alarm system, battery powered travel alarm clock, etc. that was put away with the batteries still in place might be stored somewhere (like a suitcase, in an adjacent closet?)old smoke detector, or a really older style CO alarm that doesn't have a user-replaceable/accessible battery, and the dull, intermittant chirp because the batteries are all but dead - same might even be in the upstairs or common wall neighbor's unit - or in a common area, or attached garage/systems closet area. A neighbor's parakeet or other pet bird, or a cricket could be hiding in any of those areas as well. Finally, does your parent wear a hearing aid?

It turned out to be an old water heater with an overflow sensor that was beeping, very odd. Thanks to all that replied.