Was wondering if there're one-way return vents that I can install to
allow
the cold air to be sucked into the vents when the blower/heater's
running and
NOT allow cold air to escape the other way (back into the room) when
the blower's
idle.

For some reason, when the blower's not running, there's cold air
coming outta return vents - a whole lot of it. Figuring out where it's
coming
from is a whole different task.

"For some reason, when the blower's not running, there's cold air
coming outta return vents - a whole lot of it. Figuring out where it's
coming
from is a whole different task. "

Yes, but that's what I would focus on, not trying to cover it up with
some sort of one way flap. If cold air is blowing without the furnace
running, what do you think happens when the blower kicks on? It's
gonna be sucking in even more of that cold air into the furnace system
and wasting energy.

There has to be a hole in the retuair system.Your right about finding
it may be a problem. Depends what type of home you have and what kind
of system. If your home is like a double widemobile home you probly
have a package unit A/C outside with 2 main ducts hooked to it. It may
be as simple as one of the ducts comming unhooked.If you have a frame
or block built home you probly have a Split system with a combo of
metal/fiberglass board and flex ducting. depneding how its ducted It
could be very easy or very time consuming to find your leak. Bottom
line is you have to find it!