Just watched a video on the Internet where a person puts tape up the centre of a metal wall vent on the solid part. They then get a pen and putt small lines on the tape to correspond with where the vent lines are. Has anyone else seen this & why are they doing it? After drawing lines on the tape in the centre of the vent they simply pull it off. It says to always do this before you leave an apartment. Can someone explain it to me why they would do this as it don't make any sense to me. Thanks in advance.