what could it be?

Charge the battery then do a drain test by leaving the Earth lead off and connecting an ammeter/multimeter set to measure amps between the earth post and the earth cable.
Do not turn ignition on, make sure everything is turned off including interior light (close door).
It should read no more than 0.3 amps.
If it's less than/near to that then the car's OK, it's the battery that's no good.
If it's more than that start pulling fuses until you find the one that makes the amps drop.
This will show which circuit is faulty.
Note- remove one fuse at a time, check meter, then refit fuse. Dont just keep pulling them out or you end up with a handful of fuses and no idea which one goes in where!!
A half competent mechanic would have done this for you.

Does it have a clifford alarm fitted?
Start a new post if you need more info.

ged333
August 2009

once knew of a vehicle with similar fault and it was a relay that was stuck closed -so check all relays are [cold] off when ignition is.

k
August 2009

Charge up the battery with the alternator unplugged, and see if it holds its charge. If it does, then the battery is ok. Reconnect the alternator. If the battery goes flat again the alternator is faulty.

John
August 2009