Occasional random chimes from 1970s-era mechanical doorbell?

I have a Zmodo Greet wired into my ca. 1977 house and its mechanical doorbell chime.

For some unknown reason, every once in a while (meaning any random time) the mechanical chime gets triggered and one of its solenoids dings the metal bar that produces the chime. In the cases I’ve seen, there is no motion passing by the camera nor anything else to trigger it.