Oof! Guess I should have looked at that.

Not sure if this meets your needs but you might check out DoorBird. They claim to work with several NAS solutions and have an API, as well as the usual phone- and tablet-notification and communication through their own service.

You are indeed, but it points to a fallacy in the original question. It’s not universal basic income if it is stipulated that you have to do something to receive it.

I used to calibrate industrial gas detectors. They generally have an active air sampling path with a pump and a sensor in the gas flow path, and we would calibrate them by flooding the sensor chamber with a test gas at a known concentration. For the type of sensor you have none of those conditions exist (no controlled gas flow path and no pump) so there is essentially no way you can accurately test their response. You essentially have to trust the manufacturer to have made a good product. The one thing you can do is look for an install date and (hopefully) an expiration date, if you can inspect the back of the detector without setting off any central alarm system. If you really don’t trust the owner’s sensors, you can always buy and use your own. Just make sure you place them carefully according to the manufacturer’s instructions.

Also, your detectors are probably combo smoke/CO, but not natural gas. You would smell the odorant in the gas long before the concentration becomes dangerous so a detector would be redundant.