the car doesnt have keyless does it?
Otherwise, it sounds to me like perhaps you should try the spare key. It may be a faulty transmitter that is constantly sending a signal?
if was keyless, not necessarily, but as you need a key, I would expect it should. It could be the Central Locking module is the problem. do the doors lock ok if the battery is disconnected...?
very weird, is it only when the car is with no keys, or only when the keys are in the electrical position / ignition on?
sounds to me like one of the doors isnt locking. If the bonnet lock, front door locks, or the boot lock fail to initiate, I believe none of the doors lock.
Disconnect the battery and try lock the doors individually and see what happens...
Sounds to me like one of the front door modules have gone faulty, the only way to test for this is to disconnect the front doors, one at a time and try locking the car with the remote, if the lock bounces back open with the passenger door disconnected, but not with the drivers door disconnected, then it's the drivers door at fault.
Likewise, if the lock bounces back open with the drivers door disconnected, but not with the passenger door disconnected, then it's the passenger door at fault.
could be a wiring issue, I suggest your best placed either seeking an auto electrician, or a dealership! it will save you money and headaches in the long run!