What is the little plastic container very small that has a metal bead in it?
This small clear plastic container with a tiny little metal bead is a PEMF tester, a device for testing PEMF (pulsed electromagnetic fields) function.
We cannot feel PEMF, but if the metal cylinder moves in the container when the PEMF function is ON, it means that it works fine.
This movement is the reaction to magnetic field pulsing. It is stronger when it is close to PEMF coils (3 coils in the upper part of the mat). The movement strength of the metal bead will decrease with distance.
A similar PEMF tester indicator function is built into controllers, and you can also see running/flashing lights there when PEMF is ON.
Please, check this video:
You may also make such a device yourself, just put the smallest piece of metal like a staple into a plastic disposable cup and use that to test PEMF function.
When you press the PEMF button, the function is activated for 20 minutes each 2 hours of mat active session.