Tire pressure will drop with dropping air temperature but not enough to completely flatten a tire. To completely flatten a tire overnight would only happen if there's an air leak. The leak could be caused by a puncture, but since you say you can't find a puncture (and you removed the wheel from the car to inspect the entire tire thoroughly while off the car) a leak can also be caused by a leaking valve stem or even a leak around the bead of the tire if the wheel happens to be bent.

yes it is possible I also recently had a real cold snap and lost nearly 5 lbs in all four less than a year old tires. If the tire is older and worn the cold air may cause it to go flat also if worn filling it up to or above the psi rating may also cause issues.

the valve stems have a tiny little rubber seal in them and in cold weather they get hard and don't seal well so replace your valve stem with a new one and refill the tire and you wont have anymore leaks unless there is a nail in the tire. its fairly common for valve stem not to seal anymore when disturbed after not being messed with for a long time. even in summer sometimes the rubber seal just stops sealing and this is why you should always install brand new valve stems with every tire change so the valve stem and its valve core are both fresh new rubber and seal well.

to test it use dish soap and fill the opening of the valve stem after refilling with air and watch the bubbles come out

A drop in temperature would reduce your tire pressure, but you'd need a gauge to detect it, and it would affect all four tires equally. You've got a leaky tire, valve, or bead seam.

No. if it was the weather, all would be flat.

BTW, fully inflating tires for snow is counterintuitive. If you decide to do one or the other, it would be to deflate.

Hi yes valves do leak it is a common enough problem so get it checked and changed.

Completely ?? No. "Softer" - Yes. Like most everything else, cold air takes-up less volumn then warm air.

I wonder what you would blame a flat tire on if it was eighty degrees.

A few lbs sure,,,, flat no get it fixed

the air will expand when it warms up...

A few days ago, I completely filled up my tires because we were suppose to be getting some snow (turns out we only received maybe half an inch, and it's completely gone now. Go figure). Well, on Tuesday, it was probably the coldest of the year so far; the high was 22, the low was around 3. I drove that day and everything was perfectly fine. However, I wake up early Wednesday morning to find one of my front tires completely flat. I've never had anything happen to me like that. I can't find a nail or anything, and that tire has never given me problems before. I just don't know what caused it to go flat, except maybe the cold weather. Any ideas?

Thanks.