At the NHL in Holland where I study I heard teachers are often frustrated because 20 times a day students and other people come into the room, to disturb them and take them out of their concentration, to ask if they have seen a certain person, that day, or if they know where that certain person may be at that moment.
In some cases (as a student) you are looking for a person you don't even know what he or she looks like.
I encountered a solution to these inconveniences, on Lulea Technical University in Skelleftea (Sweden) where I'm doing my graduating project.
So simple but yet so useful.