It has three legs and small (free not attached to anything) magnet inside.

Mod edit: Moved image to AAC.

 

What does the other side look like, and how do you know what’s inside?

For that matter, how do you know it’s a sensor?

 

Accelerometer?
Direction Change Sensor?
Low Power Angle Based Motion Sensor?

Or maybe it's a gyroscopic stabilizer?

Just guesing

 

Other side has three terminals PCB mounting style.
The seller opened top from side with a sharp pointing pin.
because I bought as sensor in a lump-sum trade with other sensors.
I assume three legs for a such small device with floating magnet inside can't be anything else but some kind of sensor.

https://www.dropbox.com/s/se8nwyz6ylsq8fq/20171102_203134.jpg?dl=0

 

No clue actually.
But I love a good mystery.

Here's it is without needing a link....

 

Some kind of noise-maker? Coil in the plastic base, ac applied makes the free magnet rattle around in the 'cup'?