I live in an upscale condo as well. It does happen.
The concierge can't man the desk every minute. Residents are instructed not to hold the door open (unless of course, you know the resident) and to provide their buzzcode to food delivery people (initial rule was for residents to come down to the lobby to pick up their food, but that changed to full delivery). Concierge is instructed to stop anyone they see who enter the lobby without using their fob - if that person is not recognized as an owner/tenant. Delivery people/guests who are buzzed in still need to check in with front desk.
A lot of times as I'm entering the main lobby from outside and I see someone I don;t recognize loitering at the entrance/intercom and they follow me in when I open the door, I'll pass front desk to see if they'll stop to check in. If no guard is there and they don't stop and keep following me to the next set of fob activated doors, which allows access to the entire building, I'll pause and politely mention to them that guests (or deliveries) are required to sign in. Simple as that. I won't push the issue any more. Some actually stop, but quite a few also keep following me.. At that point, I'll just turn off to the mail room or stop and sit in the lobby to check my messages/check emails. Usually over those couple minutes, the guard shows up and sees the person hanging around and asks if they need help/tell them they need to sign in.
This also happens in office buildings as previously mentioned. On our floor, we had a full height glass/metal turnstile installed - only one person can enter per card swipe.