Single Beep When Car Is Parked

This started today: I park Prius (2007) in my garage. Turned it off. All of the indicators are clear (not illuminated). About every 20-30 seconds after that, the car beeps. It sounds like it's coming from under the hood as it's quite easily heard outside the car but muffled on the inside. I have replaced the batteries in both of my FOBs. The car can be locked/unlocked via the FOB, the car starts up just fine so what's going on. I haven't seen anything on the boards to fix this and there's nothing in the manual from what I can tell. Any ideas here?

Here's video of the issue.

 

Thanks for all of the info! I ran the battery level test (from PriusDIY) getting the car into System Check mode. The initial reading is 12.3 volts in Vehicle Signal Check. Adding current load displayed a level of 12.0V. Turning the lights on and the Rear Window Defroster dropped the level to 11.5V. Turning the car on and the level went to 14.0V. According to PriusDIY, all of the reading are on the low side of "good" with the exception of the first 12.3 reading which is a tenth of a volt low. So, is that indicative of a bad battery?

 

Right now, the sound has stopped. I'm guessing you're right about the battery and will monitor and replace when the battery level drops to a more "bad" level.

 

After about a month, the battery levels are still about the same and no chirps! I am wondering about an issue I have with the fan where once in a while, the fan takes a couple of minutes before it kicks on. This has happened maybe twice in the past year, but this past month, it has happened about three times. Is this related to the battery issue?

 

OK, it's been a few weeks. I did start to have an issue where the heater fan would take a few minutes before it kicked on. I replaced the accessory battery and I haven't heard the beep anymore nor have I had any fan issues. So I believe my initial issue can be chalked up to another weird effect of a marginal battery. Thanks to all who helped me out!