Air leaks from alloy wheels are not uncommon.

1. Porosity in the alloy casting is some times a factor in air leaks.
* If porosity in the casting were the issue then your Sporty would have seen air leaks from day one. Not only during the colder weather.

2. In most cases when an Alloy wheel leaks, it's caused by corrosion at the inside edge of the wheel rim where the bead of the tire is seated.
* The issue of corrosion at the bead tends to occur on the second, third or fourth set of tires mounted on the vehicle. As the corrosion builds up, it's more difficult for the bead of the next tire mounted on that rim to seal fully.
* Thoroughly cleaning the buildup of corrosion from the bead area on the rim is an important step whenever a tire is dismounted or mounted on an alloy rim. In addition, tire shops can apply a special sealer to the rim area to help seal the tire bead and prevent future air leaks.

Obviously when temperatures change so does the expanding and contracting of metal objects; like wheels.
So if you have corrosion then it could expand in the cold and cause a leak.