Well, gunfire draws the attention of more than zombies: as they reach the 4th floor landing, a male voice calls from above. "Whoever's there, hurry up. The racket you're making is like the dinner bell to those things!"

Monica puzzles at the voice. "Steven? Is that you?" The man peers down. "What the hell are you doing here, Monica? Nevermind, hurry!" He beckons them urgently, holding the door into the 5th floor open.