What's the car trying to tell me?

I've had my 2012 Prius v Five for about three months, and I'm very happy with it. Mileage is great, and I have no issues with the car. Very well designed. However, I've noticed a two-toned (high-low) "beep" after I turn it off recently (takes about five seconds after I turn off the car before I hear it), and I don't know what that means. I have been hearing it just as I start to exit the car. I've looked through the manual trying to find any reference to it, but no luck so far. I was thinking that maybe it has been doing this since I bought the car, but I never noticed it before. I don't know, so thought I'd post this here. There are no warning lights illuminated in the instrument cluster, so it can't be a big deal. I just don't know what the car is trying to tell me. No windows are left down. Any idea what the beep means?

 

I should have mentioned that I checked that too, and "no" the headlights were not left on.

 

That is possible. I do have my phone connected to the nav/audio system. You are probably onto something with this suggestion. I can switch my phone off before shutting down the car and see if I hear the same thing or not. I'm guessing this will take care of it. Thanks!

 

This did indeed seem to be the source of the beeping noise. Thanks again! Great forum.