Vent leak

I have a 3 story house that is heated by forced air gas. Recently I noticed the top floor does not seem to be staying warm.....however, on my way up the stairs to the top floor I noticed a warm spot on the outside of the wall.
It is very specific to an aproximate 14" diameter area. It is NOT hot...just a warm spot. My theory is that there is a seperation or some sort of leak in the ducting. I thought I would carefully cut a small hole in that area and see if there is any air flow. Make sense? Any other theories?
pdxadd9.