There's a 32kHz RTC oscillator in the MCU, which can be programmed to periodically wake it up as well.

Without analysing the firmware you wouldn't be able to confirm for sure what it's doing.

I suppose one exercise for those inclined would be to reprogram it to listen (perhaps only during the daytime) and see how long it lasts. You wouldn't need high fidelity so a low sample rate would be enough.