This sounds like the Anti Theft measure by Peugeot and what it may be is the Radio thats in your car now is not originally from that car.
What the radio does is when switched on it checks the VIN number thats programmed into the radio to that whats in your ECU. If they don't match then you'll get a beep every 5ish seconds as it presumes it's stolen.
They can be re programmed with the VIN of another car no problem, try your local radio decoding shop first as i can imagine Peugeot will charge a fortune for this service.
Good Luck.
Pro
June 2006