Back in January I set up a new-to-me set of Reynolds wheels with tubeless ready Schwalbe tires. It was a little challenging to get the front sealed but the back sealed on the first try. In the several weeks after I set it up, the back only lost maybe 5psi every couple weeks. But the other day I was planning on taking the bike in to Vegas so the afternoon before I made sure I topped off both front and rear tires, set to 100psi, then put the bike on the rack on the back of my car for the night.
I wake up the next morning and go outside to leave and that same back tire that's been holding air this whole time is flat as a pancake!

I have a small compressor in the garage and it still had air in it so I refilled the tire. Still holding air today.

I checked the valve when I noticed it was flat and it was tight, not like I forgot to retighten the valve after topping off the pressure. So what the hell would cause it to just go flat overnight for no apparent reason, but then it's held air for the last two days since?

Just weird.