Pour tuna water on a carpeted area near a heating vent.
The smell of fish is generally unpleasant when outside the context of a meal. When placed near a heating vent, the warm vapours will assist in the distribution of the scent. There are many people who do not like the smell of fish whether they are eating it or not.
The office gossip will run all day as staff members not only complain about the smell, but try to figure out the mystery of where it came from. It is not hard for people to waste time in an office. They just need a reason to do it.
If the smell of fish is not alarming enough for coworkers due to their own insensitivity to the smell, try alternating in malt vinegar. Ironically malt vinegar is known for its pairing with fish and chips. The smell of malt vinegar, however, is more unpleasant to some than fish. For those who are not familiar with the unique pungent odour of malt vinegar may have difficulty placing where it is coming from.
If any employees are known for fishy smelling lunches, try this prank on a day they are on vacation.