How to make sure a dampen floor and wall is caused by a leaking pipe?
There's no way to positively say whether it's the pipe or the pressure washer.  The water meter readings may, or may not, be meaningful - that water could be going somewhere legitimate as Daniel Grissom said, and it's a very small amount of water.  To check the pipe you'll have to get access to it, which means tearing up the tile.  So what's more likely - a pipe started leaking at the same time that you pressure washed the patio, or that water got into the wall while you were pressure washing?  If it was me, I'd get several fans blowing on the damp area for a few days to see if it drys up.  If it makes no difference after a week, then maybe you need to dig into the tile.