I'm starting to suspect someone let the air out of my tire.

You can spot check your wheel/tire with a bottom of Windex. There is just enough surfactant in there to bubble up if a leak is present. If no leak you have a minty fresh clean wheel and tire.

If you think someone is messing with you, you could take a Sharpie and mark your valve stem and cap. Odds are if someone is messing with you they wont align the marks back up after their deed.

 

Gosh: Everyone must have been pretty bored when you first posted this, considering the number of replies you received.

But as a contribution to the knowlege base, I kept having a mysterious super slow leak on one Prius tire that we could never find. I ended up replacing the tire after about a few months of hassling it.

Then after about 18 months, the same deal with the new tire. I took it in about three times, never could find a problem. Finally, fourth time, I told them to dismount and then remount. That worked. It's been about four months and no problem.

I am thinking there might be something about that particular wheel/rim, or whatever it is called. I would not be surprised if sometime in the next year, that tire starts to lose air again.
If it does, I might consider replacement of the wheel/rim, but considering how much $$$$....I might just live with it.

The valve stem is one of the first things I check, if I see no other obvious signs of trouble. Bought one of those handy stem tools some 30 years ago when I was in my 20's I think it cost me 50 cents or something.

Gave it away to my daughter about a year ago and bought another one for myself. Daughter used it to fix a slow leak on one of her tires.