Rode my bike 18 miles two days ago and when I garaged it both tires were at normal pressure.
Woke up yesterday morning to ride again and rear tire completely deflated.
After inspecting it I found no visible damage or puncture. The bike was in the same position I left it in so know it didn't fall over. Plus the valve cap was still on(yes, I am one of those people lol)and screw under it was tightened down in normal position.
Inflated it to 90lbs and test rode it for a mile with no loss in pressure.
Didn't ride it longer than that though for fear it would blow on me.
Checked tire again this morning and still at 90lbs.
Any ideas?
Can anything other than a piece of debris from the road cause an overnight flat?
Makes no sense to me but I am mechanically challenged so figured I would ask here first.

Pump up the tire to the max allowed and put the wheel partially into something like a pond, creek, or swimming pool. Rotate the wheel in the water slowly to see if any air bubbles come streaming out. There might be some air bubbles on the side of the tire or even coming from the rim. Not talking about those. If air is leaking out, it might be where the leak is especially if it's from a cut or hole in the tire. Or not. But mark the spot anyway.

The next step is to pull out the tube and partially inflate it. When removing the tube, keep track of where it is in relation to the rim and tire. Put the tube in water and see if you can find a leak. If not, wiggle the valve stem some to see if leaks near the rubber. If you do find a leak, mark it. I like using chalk. Then compare the leak to the position on the rim/tire. Find what caused the leak, such as a shard of glass or strand of wire, and remove the problem. Remember it might even be the rim or rim tape that's the issue. Next decide whether to fix the tube or replace it.

One problem with super slow leaks is that they often don't show themselves at the low pressures when just testing the tube alone. That's why I recommend the extra step of testing with the tire fully inflated. If it's a fast leak, just the tube testing alone is enough.

Bath tub works in event you don't have pool, creek or pond. I've seen someone not tighten the valve closure enough and then screw the cap on too tight which depresses valve and air leaks out? If by any chance you used Co2 to inflate tire it tends to leak out much faster than air.

The few times that this has happened to me, it ended up being a problem around where the valve stem goes into the tube. Apparently the way the valve is rotated on the bike when stored affects whether it leaks or not. If you inflate the tube and wriggle the stem, it should be apparent.

Good luck.

Thorn resistant inner tube are heavier but all that extra Rubber make then hold air for a Long time.

NB higher you inflate your tires the greater the tendency
for the air pressjre to escape to seek equalization with the Ambient Pressure by escaping thru the pores of the inner tube ..

Yes, while it's rare, I've had tubes fail at the bonded valve joint. Early in this process it can be intermittent, with the side of the rim's valve hole putting the finger in the dike. I'd enjoy it while it lasts, but ride with a spare and if it happens again, bite the bullet and replace the tube.

Update:
After I posted I didn't do anything to the tire and rode the bike 32 miles with no issues at all today.
Tire is still right around the same pressure as when I first filled it after discovering the flat yesterday.
Isn't that kinda weird?

Shhhh! don't say that anywhere near the bike. Enjoy it while it lasts, but keep the replacement tube handy.

probably a chunk of debris in the valve... just keep an eye on it. if it's a Schrader valve, replace the valve core... if it's presta, keep an eye on it.