Citroen C3 constant beeping

This problem has only occurred twice but has worried me a bit. when I turn the ignition on the car starts but it also starts beeping constantly, about a second or two apart. This beeping continues if I turn the car off again and the radio also stays on and won't turn off.The first time this happened i turned the car on again and the beeping stopped when the car started the second time. But last night I had to turn the car off and on again three times before it stopped. nothing else seems to work. No warning lights come up and nothing else appears to be wrong, the car runs same as usual and we have checked stuff like the oil level. I have searched the internet and the car manual for an answer but cannot find one. Any ideas anyone?