My headcanon has always been that by the time Jill arrives, there's still a group of police officers still alive and they have barricaded the majority of the police station. And Jill assumes Marvin is already dead, as she's in a hurry. After Jill exits, pursued by Nemesis, the remaining officers try to enact their escape plan, but all die as zombies and/or Irons destroys the barricades. Then Irons activates the electronic locks after everyone's already dead, probably to protect himself. Then some hours later, Leon and Claire arrive, Mr. X is dropped in, Irons kills the mayor's daughter and Ada arrives in search of the sewer/lab entrance.
I'm only going by the notes Leon and Claire find throughout the station left by the remaining officers, so a lot is just assumed on my end.