@romp: No, that's not it. The sound I'm describing is clearly not the rapid beep beep beep that is used for warnings like for pressing both pedals or opening the door before shifting to "P". The three sounds are substantially slower (maybe half the rate) and much more musical, like the simulated chime sounds heard on some other cars.
@rlang59: I'll try that. Perhaps my fob is being pressed as I enter or exit the car.
@Larry93535: I hope it's not static somehow causing an otherwise unused mode of operation. If it were, I'd worry about what else might be happening.
@Archer: This has happened while the car is stopped and nothing is moving near the bumpers to set off the parking sensors. Also, I didn't see the on-screen display that automatically shows the parking sensor readings.
I'll test rlang's idea and report back. Thanks.