Is it by any chance the seatbelt warning? I used to always put my backpack in the passenger seat and every so often while I was driving the car would beep and I couldn't figure out what it was as I'm looking at the displays, etc. One day when I was beeping I looked at the clock in the front that I don't normally look at and noticed the seatbelt lights.

I've since gone to putting my backpack on the floor now .