Honking sound in Park when brake is applied

Bought 2008 Prius over a year ago. Immediately after I took it in for oil change service around 10k at Toyota dealer the honking sound started. It occurs when in Park & not on, depress brake pedal and 3 short honks sound. It is the horn, not a beeping inside. If you're outside standing in front of the car you can tell it's the horn. This occurs consistently every time when in Park & brake is depressed.

After a trip to Santa Monica, we had to valet the car to park it overnight. Mysteriously now the honk was gone and did not come back, until . . .

I ran out of gas on the freeway (oops!) and car completely died. After adding some gas & starting up again, the crazy honk-honk-honk is back.

Told same Toyota service about this yesterday when I went in for oil change again. They claimed it's an after market alarm device probably installed by dealer where I bought it, and they have no idea what to do about it, why it started when they changed my oil in the first place, why the Santa Monica valet guy could make it go away, and that since it's "not Toyota" wished me luck on the way out.

Any ideas?

 

Thanks for all your input. I made an appt with service at dealer where car was purchased this morning. Just when I finally decided to attack this problem that has been going on consistently for 6 months, joined this forum, posted, then made appt - what do you think happened???

The triple-toot stopped!!!! Completely. I'm not kidding. My husband and kids will all attest to this. I don't know why. Just stopped after grocery shopping.

Cancelled appt since it's not happening now, but if it comes back I'll take it in (& will definitely check out battery scenario as well as alarm. . . )

 

FIXED!!!

Took car into dealership where I bought it. They install alarms on all cars, then deactivate them if you don't want to pay for it. The alarm deactivation chip was not making a good connection, thus the triple-honk. The device sits just under the steering wheel/dash, and all I had to do was push it up to snap it in better.