The slate-gray light of a wintry morning hung over the 7-Eleven parking lot just after sunrise yesterday when a murmur rippled through the crowd of men gathered there: "Look, they are coming." Heads turned in unison to a dozen people moving toward them on Elden Street in Herndon. Although the men's clothing -- work boots and bluejeans -- revealed them as day laborers, the new group wore warm winter coats and snug-fitting gloves and carried cameras with long lenses, a camcorder, a couple of walkie-talkies and a clipboard list of license plate numbers collected on previous visits.