citroen c3.....constant beeping

my daughters Citroen C3 (55 plate).....when she parks up and switches off her ignition, the radio does not switch off there is a constant beep (every 2 seconds) and the display shows 740 and then we have to switch the ignition off several times before it stops.....I'm lost for an explanation can anyone solve this mystery before i take the car to the garage

Citroen C3
1.1 manual
2005