Doorbell rings no one there

Our doorbell has rung twice in the last two weeks and no one is there. The camera picks up no one. The ring sounds strange too. The camera is all charged. This has never happened. We just can’t figure out how this happens. Advice???

Could be a prankster.
Can someone run up to your door, ring the door bell, and run away and only be in sight of your doorbell for less than say 10 seconds? If yes, then that’s a likely scenario as it takes a few moments for your doorbell to wake up to take video giving a prankster plenty of time (provided they aren’t in sight for long).