This is a holder for the remote control for interconnected smoke detectors sold by OptiLife
https://www.optilifesafety.com.au/
It allows you to mount the remote on the wall so you don't lose it.
It also helps it not get lost when used in rental properties.

There are three designs. A small one that is about the size of the device, and one that has a panel behind it to allow easier mounting to a wall, and the panel one with a cutout to allow easy removal of the remote.