My Mazda CX5 started doing this recently, but I always have my phone on the seat plugged in to the cable for Apple Car Play. I notice today that it didn't to that but I had the phone in my pocket. I think you solved my problem!
I’ve had this problem occasionally happen when I put a light bag of chips on crease of the passenger seat (it’s only 5 oz and doesn’t happen every time). This is the only item that ever causes the issue, so I’m perplexed. When I remove the bag, the chime stops. Do you have any idea why this could be happening?