I don't know the answer to that. If they haven't DONE anything, they could be picked up for questioning, but unless there is reason to believe something deeper is going on, I don't think they can be held. If they had run away when approached for questioning, that could give the authorities a reason to hold them on suspicion, and perhaps a reason to ontain a search warrant.
Also last week, a non-english speaking intruder was caught inside the John Amos Power Plant in WVA. The lone security guard was an unarmed female who tried to wrestle him to the ground. He got away by boat. Have heard nothing since. Lots of stories. No resolutions.