Why do i feel cold air coming out of homes furnace/heater vents?

I have a modular home with a propane furnace. When it's not running for awhile...we noticed a cool breeze coming up thru the vents on the floor. Why would this be? Does it come thru the furnace/fan area or could there be duct work with a opening somewhere. We are on a slab foundation with only about a 2' "barely" crawl space too (sucks yes). Wondering if i need a crew with a snake camera to scope it out...