Prius chirp

My 2006 Prius has an intermittent chirp that sounds somewhat like tapping the horn softly. It appears often just after I turn on the power, when I stop at stop lights or in my carport, and sometimes after I turn off the power. Usually there is a single chirp, but there have been as many as 5 in a row. The dealer has no idea what it could be and it is intermittent so that it is difficult to track down. It has been present at least a couple years. I haven’t noticed this at highway speed, but that might be because road noise masks it. Any idea what this could be?

Click This Link And See If This Sounds Like It.