It seems that the Overwatch have already infiltrated the building, and are busy fighting their zombified colleagues. Not so busy that they don't turn and fire on Alyx and Gordon, of course; that would be too easy.

On top of it all, Alyx can see trip lasers crisscrossing the room ahead. "Think you can set off those tripmines from here?" Alyx calls over her shoulder as she ducks behind a support column for cover.