years ago, I ran a several year D&D4e campaign that went from 1-20th level. The most frustrated the group ever was, was when I told them that they found a latrine room and a pit below it. Of course the wizard threw detect magic on and I told them that there was a faint glow of magic coming from everything in the below room. They spent, in real time, over an hour searching the shit pit, growing ever annoyed that they didn't figure out that the shit was faintly magical because the troops in the keep had been eating exclusively magically summoned food for months.