Yeah, like Chuck i've heard of that happening before with Peugeots and Citroens...but it is extremely rare.
It is possible to pick up the signal emitted from a key fob and copy it too, but if a thief was going to that much trouble (and expense) to get into a car it would almost certainly be to steal it.
What year 306 do you have? From what i can remember the majority of the 306 range have 'pop-up' style locks on the top of the interior door trim, visible through the door window. These are incredibly easy to open! With the aid of some plastic parcel ribon, or sometimes only a wire coathanger, the locks can be turned in seconds.
Look out for signs of this having been done by checking the pop-up lock inside your car for signs of tampering (scratches, spots of superglue, etc), and checking the outside of your car where each of the doors shuts flush to the chassis - the ribbon/coathanger/whatever will be fed down through this gap and more often than not there will be small signs of tampering.
If this is how they are entering your car, the only real solution is to cover your interior locks with something to prevent them being reached, or invest in a motion sensor alarm that will trip if anything is sensed moving around inside your car.
ps. don't worry, i'm not a car thief...i was in the motor trade for a long time and used this trick myself countless times to unlock cars when the keys had been locked inside or lost!