Well hell... if I know my ability has been resisted, then why would I stop attempting to detect until I was fully successful? And if we're just going to repeat detection attempts infinitely, why have a defense at all?
Because you risk making that Vampire Lord/Fairy/MIB
irritated. In this case, as in life, there is nothing stopping you from endless attempts to violate another's privacy except social decorum and fear of repercussion.