Continuity mistake: When inspecting the plane for damage, the propeller changes position without being touched. The side by Alice goes from the 8 o'clock position to the 10 o'clock position. (00:34:30)
Continuity mistake: The bullet holes in the doors the two Alice clones dive through near the start are constantly changing place. (00:11:05)
Continuity mistake: During the initial fight scene, one of the Alice clones takes cover behind a pillar, which is fully intact. When the shot cuts to show her and the other clone, there are three bullet holes that have mysteriously appeared in the pillar. (00:11:10)
Answer: Based on the sounds to go with the scene, I'd say it is zombies taking the guards out. However, considering following events it is more likely that Alice and her friends have something to do with it.