I've found that if I lose any pressure over the winter, all 4 tires lose similar amount of air pressure (e.g. 3 lbs of air per tire). If you have only one tire losing air to the point of flatness, and the other three holding, but losing less, you have a leak in that one. Fill it with air, spray with soapy water all over the tire, and you should be able to see the leak. Could also be the tire valve stem.