OK I'm stumped.
I have a wire wheel with a fairly new Dunlop tire and tube mounted (it's on a sidecar). Three days ago I found the tire totally without air - not even 5 lbs. I checked the valve stem plunger (which seemed fine) and refilled the tire. As of right now it still is holding the pressure from three days ago. I checked the wheel several times for a nail or other object and can't find anything suspicious. Since it's holding pressure, water immersion would seem to be of no help.
The only thing I could come up with is that someone let the air out of this one tire and that is extremely unlikely - the bike is garaged.
Anyone out there have a similar experience. I'm ready to take the wheel to my local shop and have them check everything, then replace the inner tube.