Different sounds?

My ring doorbell makes three unique sounds. One is when the button is pressed. The second is when someone gets near the doorbell. It's the third sound that I can't figure out. No package, It can't view the road so it's not a car. No animals. Someone suggested it might be the doorbell telling me that it is charging, but when it did that before upon the setup, it sent me an email not a chime. And I haven't gotten any more emails about the need to charge. Anyone know what it is?