Another *What is this*

[Jerry Simon 19]

High-rise condo heated by an in-wall, two-pipe type of fan-assisted hydronic heater.  Right above the wall supply register, which is right above the heater itself, there is a small plastic box that resembles a doorbell enclosure from the 1950's.  Inside this box, there is a sensor(?) thingy wired with low-voltage wiring (there are six wiring strands here, but only two of the wires are connected to the sensor thingy).   The sensor is made of white plastic, and is about 1" x 2" in size.  Neither the box nor the sensor are labeled.  Anybody know what this is?   (It isn't a doorbell, by the by.)

[Marc 39]

I'm guessing temperature sensor for the thermostat or flow control solenoid.