2017 Toyota Prius C has intermittent alarm beep problem

Would be much appreciative if anyone has any ideas. I've taken it to a few mechanics with no luck. Included a video for reference.

Sound description

• Car intermittently makes ongoing beep/alarm sound
• Usually happens during my morning commute about 20 minutes in, but sometimes has happened earlier
• No light on dashboard
• Rarely happens in the afternoon on the way home
• It’s the same sound it makes when the trunk is open and the car is on (and in drive I think)
• Usually if I pull over, turn the car off, and then turn the car back on, it stops immediately (but then it can return)

What I’ve tried

• Re-greasing the trunk latch
• Making sure there’s nothing on any seat (i.e. seatbelt warming)
• Slamming trunk shut repeatedly (this seems to work sometimes but unsure)
• Topping off fluids

What else I think it could be

• Aftermarket alarm messing up (car bought used and has an alarm that has supposedly been disabled)
• Need new 12v battery
• Fuse?
• Wiring?
• Something related to cold weather (as it happens in the morning) or how long it’s been sitting (about 13.5 hours) but not sure because it usually doesn’t happen right when car starts (about 20-30 minutes later usually)

I have a video link but am not able to share it until I hit 4 posts, so will update when I do.

 

I considered taking it into a Toyota Service Center, but they said it would be about $275 to check it out, and if they can't recreate it, then they're not really sure how much help they would be.