2007 Prius powers back on after power off

Recently our Prius has had a curious problem. Sometimes a few seconds after power off, all of a sudden, we experience something like an electrical system reset. The power comes on, the CD player sounds like it is moving CDs around, and the display comes up and tells us this is a new car. We lose all our radio stations and navigation system is set back to factory settings. This has happened about 6 times in the last month. Took it to the dealer today and they can't find anything wrong. Of course it didn't happen when they tried it. They said there are no codes being set. Tell us to bring it back if it happens all the time. About 2 months ago we were rear ended at a stop light- the damage looked fairly minor but it was $4K worth of repair and they mentioned something about having to do a lot of testing on the electrical system to see if it was working properly- so this may or may not have something to do with the problem. Any ideas? I don't want to get stuck somewhere because the electrical system is not connected properly.

 

Thanks for the suggestions- I have checked the connections and they look ok- no corrosion and the ground wire connection is tight. Also ran the battery check and it checks out ok.
and no water (yet!) around the spare tire either!
and, the problem happened again yesterday... trying to narrow down when it does happen- like do I still have my foot on the brake or don't have it on the brake when it powers back on, etc. 4 drivers in the house and we've all experienced it at least once!

 

Still happening and with more frequency now- - sometimes it seems like it loses everything (navigation history/settings, radio, clock) and sometimes only clock and radio while the navigation history is still there. Yesterday after I turned the car off I sat there and decided, yes, it's off- got out of the car and attempted to lock the door- started beeping at me- wondered if I left my keys in the car- no, have them in my bag- and finally figured out the car had powered itself back on again after I exited! Still have not figured out any pattern to this other than seems like a short drive (15 minutes or so) is most common. My husband said he noticed a couple times that on pressing Power off, everything went black- that is the normal blinking dashboard signal went out completely and then a few seconds later the car powed itself on again.

 

no, haven't replaced it. I did do the battery check as suggested by Richard (#5). I will probably take it back to the dealer and have them check it again and find out about getting a replacement.

 

Update- took it to the dealer and it happened again when I powered it off while at the dealer so at least one person there saw it happen. Of course the service technician could never get it to happen even after keeping it for 5 days and driving it. But, they did replace the positive terminal on my 12V battery and they said that the shop that did repairs after the accident had painted over the ground wire so cleaned that up. They hope that will fix it since the next option suggested was a whole new "electrical harness" which would cost something like $8K!

 

thanks! Saw this message after I posted my last update- I will definitely tell the dealer about it if the problem is not solved by the terminal replacement/ground wire cleanup. And obviously would 12v battery replacement as the next step if it happens again.

7/14/2009:
Problem appears to be resolved with the work the dealer did on the battery. Has not happened again in 10 days and it was happening several times/day by the time I brought it in again- thanks to all who responded- as all of you suggested, it was a 12V battery problem.