It LOOKS like an ordinary smoke detector.

This photo illustrates the pure geek in me. Some years ago, I was shopping online for some electric guitar parts and happened across MintyAmps.com (the site still exists, but has been inactive since 2010). They were a source for these really tiny guitar amplifier circuits. I bought two, and installed one in the gutted smoke detector housing you see here.

The test button now serves as the amp's on/off switch and is lit by a blue LED when the power is on. You can see the instrument cable input jack on the right edge in the upper half of the photo. The bottom half shows the components. Clockwise from the 9-volt battery at the right, we have a 1-1/4 inch speaker, the on/off switch, and the amplifier circuit board.

I also added some stick-on rubber feet to protect the surface that it sits on and to keep it from sliding.