I had something similar happening to me on my HiHy. Come to find out I was accidentally hitting the lock button on the remote FOB while the door was open. I happened to me several times, and it wasn't until I took it to the dealer for it's 5K service that we figured it out.

I don't know if it's what's happening to you, but try locking the car from the FOB when the door is open and see if that is the same beep you get.