Is this a hidden microphone or some kind of power supply?
The button on the left clicks in like a pen and an internal lever locks it into place. It will not ever release unless the unit is disassembled. After pressing the end, the internal pin makes contact with the metal can on the right. The can was broken open open during extraction and has a core of an unknown material. There is also a single wire protruding from the side. The insulation originally covered the wire. Markings on the plastic housing are NB and under the NB, a lower case letter "a". It is about an inch long. A larger, milder spring and internal pin that looked like a tiny nail that was in the square housing are not in the image.