Rubber return line from fuel psi regulator to metal return line is leaking when cold.
The spring clamps do not stay tight when cold and when the rubber expands when hot it seals.
Remove the upper part of the intake and replace the spring clamps with screw type clamps.
Had the same thing on both my Kia's. Theres fuel fumes in that line or the fumes are from the tank. You only notice it in winter because the heater intake is just behind there. When you speed up the draft blows the accumulation away, but the heater gets some. In the warm weather the clamp holds tighter.
It's pretty easy, take the 4 nuts that hold the plasic cover off, look to the passenger side of the engine, there are 2 tubes running along the top rail toward the front, with spring clamps. Replace them with flat hose clamps, you can get at Lowe's or home Depot for 45 cents each.