Some of the reports may be mistaken.
Some of the men may just be tourists or sightseers.
Some of the men may be collecting intelligence for real terror attacks.
Some of the men may want to be testing for a reaction of security or public awareness in a given area to find a weak target of opportunity.
Some of the men may be terror sympathizers who want to terrorize by the act of being seen taping targets of opportunity.
Do have any other possibilities to add to the list?