Can't explain this one at all

Just an XM being an XM perhaps? I don't know..
I noticed this afternoon the XM had a distinctly soft tyre. 'Fiddelsticks, I've picked up a puncture' I thought. And in a nearly brand new tyre too

I moved the car to get it within range of the airline and found in the 10 or so yards it had travelled the tyre was even softer.
I blew it up on the airline and put the car on high to take off the wheel with the intention of taking it to the tyre place for repair. I looked in vain for the nail or screw though and in fact spent half an hour when the wheel was off the car looking for it with a microscope almost. I searched and searched but could find nothing, not a sausage. I even made the tyre wet to look for bubbles. None. Not even the tiniest hint of a bubble or anything. I checked the valve. Seemed OK. No rim damage and I know I've not kerbed it or done anything silly with it.
Couldn't have been tampering as the valve cap was in place.
Previous to this all the tyres on the XM have been brilliant at holding pressure so it's not a porous rim or anything like that and it was Ok yesterday.
When I initially blew up the tyre I set it to 2.6 bars. Two and a half hours later it was still 2.6 bars. I put it back on the car, went for a short spin and checked it again. 2.8 bars. I've just checked it yet again some 5 hours after blowing it up and it's still on 2.6 bars.
So what's going on? I suppose the test will to see how it looks in the morning...
Bloody odd though...