You mean they don't have an exit gate?
Here in the US of A the "self service" parking usually entails getting a ticket when you go into the parking facility, and then when you are about to leave, going up to a vending machine, and submitting said ticket and plying the machine with folding green paper with pictures of presidents and other elder statesmen, or a credit card. The machine dutifly records said payment on the ticket and the time, and you slip said (now validated) ticket in the slot at the exit gate and your car is now out of jail, and you are free to go.
Now if you don't pay, you don't get out of the garage/car-park, being trapped until you can find some attendant or just give up and ram the gate (they are usually pretty flimsy) and bust out of "car jail".
The alternatives are as I saw this last weekend, having a fixed price to pay upon entering a parking facility (it was up to $30/vehicle at the Rock Concert in Napa!), and the time was "unlimited". Of course if the car stayed overnight or was abandoned, a nice friendly towing company would take possession and cost MUCH MORE to get it out of "maximum security car jail" (aka impound lot).
Now where was that nice portable jet pack when you needed it!