Zombies in the car park

Rating: 8/10 (23 December 2004)

I'm a little puzzled...
Maybe someone could help me...
I have seen the film a couple of times (and think its pretty good).
At some point in the film our heroes go into the underground car park to fix up some generator. It was at this point that there was a 'kerfuffle' (film speak for fight ;-) ) with some zombies...
However, where did the zombies come from in the underground car park? If there was an entrance from the outside - reinforcing the buses would have been impossible - as there would have been a constant stream of ghouls..?
Please enlighten me... so I can get some sleep over Christmas ;-)