Noticed
Initially, he had been walking with her in front of him but changed that as soon as he noticed that there were other people there. This in itself did not raise an alarm, but something about the girl did.
Stop To Check
They were leaving the underground parking lot when they saw something that made them stop in their tracks. What could they have seen that would make a group of teenage boys stop abruptly to do a double-take to make sure they weren’t seeing the wrong thing?