Finding them is getting really hard. Some in my building have taken to meeting clients a couple blocks away and do not post their exact/correct location. Security actually stop everyone with a suitcase going through the lobby who was given access or a key. There's even one guy who registers the guest in an official manner with building management as a "cousin" staying for a few days. I would never stay at such a vague listing but many do. Heck, one poor fellow was renting out his unit and he didn't even know it; his girlfriend was renting it out when he was away.
As with most people breaking the rules, only the obvious ones get caught before there is a major problem (bad guest causes big problems).