Buzzing noise from rear of car while off?

I'm camping in my car (2022 Crosstrek outdoor), just got back from a 2.5 hour hike, and there was a persistent buzzing sound coming from the rear of the car. Car was off? I used the star link to lock doors shortly after I left it.

I thought it might have been a battery powered fan that I could have turned on by accident and dropped somewhere, but it sounded like it was coming from outside not inside.

I turned on the car to wind the windows down for the night, then turned the car off, and the buzzing stopped.

I'm really perplexed; I had been charging a camp battery from the 12v/lighter for my CPAP, but that's been disconnected since before my hike.

I also have a bike rack on the hitch with trailer lights attached; I don't know how that would cause this but it's at the back of the car.

Why would my car be buzzing like that, and then go away when I turn it on?