As you "explored" White Grove (or you rather just managed to get out of City 17), you heard some far noises. You couldn't really tell what that was, but why don't go and check it out? After a couple of minutes, or rather half an hour, you got to the source of noises. As you could see from your lil' hideout, it was Alyx Vance! Well, she was in not good situation... Let me go, you damn- ...'cuz she was ambushed and surrounded by Combine forces. Maybe they tried to get her and then try to use her against Gordon?

Ehem, anyways... She was surrounded and restrained by few Combines. There were few Metrocops with stunbatons, few Overwatch Soldiers and even one Overwatch Elite! You should do something, no?