[Ordered to Observe] Told to "observe to your heart's content" and desperately looks up only to realize it was in the paper bag

Forced to check the pattern of stolen underwear for the Panty Professor investigation, the detective is told "please observe to your heart's content" and desperately peeks up the skirt, but sees nothing and is toyed with. The punchline reveals a clever trap: the target was actually inside the paper bag mentioned with "I'm glad you returned it but I don't want to use it anymore"