It wasn't the seatbelt warning. Seatbelt warning doesn't occur when the car is off.
It wasn't the moonroof warning either! The moonroof was closed. When I do get a moonroof warning, it shows it on the display "moonroof open" and also gives one of those "external beeps" (like when you lock the car or take the key out of the car while it is running). This beeping was inside the car.