Hang undergarments under staff members’ coats.
This prank is unnerving to anyone. It will be harder to trace back to you if you use undergarments from the opposite sex of yourself. The target may be more unnerved if the undergarments are from the opposite sex of themselves. Decide whether your gender anonymity or freaking out your target is more important for your goal.
To save money on this prank, purchase undergarments at a thrift store. They may be used, but have been cleaned. To ensure realism, wash first to remove thrift store smell. Next wrap onions in the undergarments to give a smell that will replicate body odours. Feel free to make this portion your own. Add garlic, dog food, or whatever items you feel will help to give the undergarments a questionable odour.
If your goal is to make your targets question what is going on, employ more deliberate means of unnerving them. Undergarments can get mixed in with the laundry and a case could be made that they accidentally found their way into the office. Try hanging just waistbands from mens underwear. See how that is received.
*If you do not feel comfortable using undergarments, T-shirts with inappropriate sexist or racial comments will work in their place. Sexist and racial comments can be grounds for dismissal; whereas if you are caught hiding undergarments you are not breaking the law.