The furnace vent is blowing out hot air even though it is turned off.?

I have a 40 year old condo with a Lennox G8 furnace. The furnace is in a small furnace closet in the hallway. There is a vent on the wall just outside of that closet. Even though no fans are turned on it`s lightly blowing out warm air from that vent and I cant seem to stop it from doing that. I live on the bottom floor and my condo is constantly around 80 degrees unless I turn on the A\C. Just outside my door in the condo lobby (no A/C) it is cool. Any suggestions ??