Can a US cop ask a land owner if a person is trespassing if no complaint has been made? i.e. can they ask a shop owner if a person is trespassing in their car park or outside the shop?
Yes, of course. Anyone can walk up to a shop owner and ask them pretty much whatever questions they want; police have just as much of a right to do that as anyone else. The shop owner doesn't have to answer, but police are free to ask.