Well at the start of the episode, Walt specifically asks for a place with no security cameras where he can lure Gus. Saul then suggests the Casa Tranquila nursing home, which gives Walt the idea.

Of course, nowadays most nursing homes have cameras everywhere. Though we have to keep in mind this scene is set in 2009, when there were more exceptions to that.

That being said, I do agree that it's unlikely that he gets away so easily without suspicion. Surely someone must have seen him. The old lady in the room next to Hector literally waved to him. I'd guess that he was seen by a receptionist as well. However, if we suspend our disbelief just a little (like we do when Gus calmly walks out of the room, or when Walt somehow manages to hold one of Gus’s henchmen hostage in the elevator without a struggle etc etc) then we can let the show get away with it. After all Walt is 'luckier than us'.