Tesco (in Slough) use a similar system to that which DD describes, the store is built on 'stilts' with parking on the ground level and shopping on the upper levels, they have 3Hrs free parking after which penalties apply, however I believe theirs uses a traffic light system with vehicle sensors built into the bays.
There are no barriers or ticket machines.
I can't remember the exact system as I haven't used it in a while but I think no light signifies an empty bay, once a vehicle is parked, the sensor switches on a green light and a timer. If the vehicle is not moved within 3 hours, the light turns red attracting the attention of the parking operatives weasels.