Surveillance
Visitors had to go through a series of checkpoints to get into the gallery, proper. They had to sign in, get photographed, fingerprinted and stamped.
Once inside, visitors could finally view the artwork. They could stand on yellow feet painted on the ground for the best viewing distance and angle.
Except, the footprints were merely to make sure visitors were standing the proper place for the drawings to surreptitiously record them, via a surveillance system attached to the frames of each of the drawings.
Every visitor was documented and video surveillance was downloaded to a hard drive in a secret room.