Wake up this AM and my 700X35 front tire is flat. I figured it had a puncture, so I remove the tube re-inflate it and put it in my washtub of water. I submerged it inflated and I see no bubbles, Never had a tube do this and I always could detect the puncture in the water. What would cause this? Thanks.

possibly debris in the valve.
apply soapy water after inflating the tube some, then leave it alone for ten minutes.
if there is a leak, there will be foam or a larger bubble at the leak site.
dish soap works well.

You didn't inflate it sufficiently to reveal the leak. Inflate it until it's about 4 feet in diameter and try again. And be sure to submerge the valve while you're at it.

There is a first time for virtually everything. I had this happen and other's have too. So you are in good company. I'm certain there are different cause for many of us.

Not everything needs a explanation of cause if it only happens once in a blue moon or so.

Inflate the tire while mounted and then try dunking it in the tub again. A 50-100 psi pressure will reveal a leak much better than the 3-? psi of an unmounted tube. If you see bubbles coming from the stem hole in the rim, wiggle the stem a little as tears at the base sometimes won't show a leak under low pressure or when pressed against the rim when inflated.

Great tips, I'll do some more digging with this info. Much appreciated.

This ^^.
I had a flat last year, and no matter how much I inflated the tube I could find no leak... until I submerged the (Presta) valve. I had never seen a loose valve body before. Now I own a valve tool and check all valves... even new ones. This was a new tube.

My guess is the valve didn't close. But when you aired it up to check it, it then did close. So, leak is at least temporarily fixed. I'd remove the valve core, clean or replace it and assume you have solved the problem.

Was it inflated the previous evening? Maybe you never inflated it in the first place...

We see many flats with no obvious cause at work. I agree with the well over inflation of the tube out of the tire and placed in water to track the bubbles (or bubble after a few minutes when the puncture is really small).

Some flats are from poor air pressure maintenance, the tire pressure softens over the days/weeks and when ridded under inflated the tube rubs against the tire casing interior causing what look like directional sanding was done on it. The now abraded and thin spots can bleed air too slow to see bubbles or hear a hiss.

Another subtle flat cause we see more often than years ago is the wire puncture. The wire (from auto tire casings I think) get picked up in the tire and in time work their way through the casing to prick the tube. These holes are generally able to be seen, bubble track wise. But the wire can be hard to feel for, unless you reverse the direction you run your fingers along the inside of the tire. The wire is often at an angle that "lays flat" if brushed one way but will become "upright" if brushed the other way.

Valve core issues, dirt/bent core shaft or just not well tightened fully, happen but not as frequently as other causes IME. Still it is good form to check the valve core's tightness when installing a fresh tube. Andy.

Thanks for the insights, this was interesting. I inflated the tube to 12 lbs, still no bubbles, and the tube is still inflated. The valve stem is snug, I'm starting to think the valve itself had debris or just doesn't seal.

see post #2 in this tread...
when i can't find a leak, i usually just swap in a fresh valve core.

Having caught a few wires and tiny chips of glass with my fingertips while checking for embedded debris, I'll usually check with a microfibre cloth first to reduce the chance of lacerations. Also just work around the outside of the tyre pinching the tread so any small cut opens up, sometimes revealing a sharp or pointy thing in its depth - these are the ones that remain hidden in the tread when you feel for them, but manage to cut the tube when you ride with them.

I changed out both tubes, ones that came in bike were Kenda's made in Indonesia, switched to Bontrager's made in Taiwan, the valve quality appeared better and the rubber seemed thicker. I aired up the tire prior to riding it the night before with my Ryobi electric inflator, perhaps a small piece of particulate lodged in the valve? I've had plenty of puncture flats, and tire blow-outs never one where I couldn't find the leak.