@Quynh-Chi Nguyen,Contact the company, and ask them to send you a replacement cover, i'm guessing it was making a noise and they removed the batteries-- can you ask them if they know where it is?  

RR Do you promise even when humans are permanently wired to the Machine network and being used as batteries for the proliferation of Machine consciousness?  

It has an internal battery backup.

Batteries died in the thermostat (I'm not kidding; this has happened twice with two tenants and they couldn't read the "low battery" warning on the thermostat).  3.

Used to was that if all the switches were left in the right positions and they still conduct current, you could tow a sunken submarine to recharge its batteries, raise it if there's no air pressure t blow the ballast and run the pumps to re-float it.

Damage happens, things break, screws get lose, leaks occur, mold forms, batteries die, filters need replacing. 

We switch the battery out once a year, gives us a good reason to come over and just check stuff out. 

We send someone in 4-6 times per year to change filters (furnace and HRV), batteries (thermostats, detectors, etc).  

The first floor that has a second floor above is considered not accessible and therefore a battery operated will suffice. 

The other 3 were for battery and burglary but the last names were different.