Can you say "slow leak at the valve stem?" It's happened to me. The act of checking and filling the tire clears the stem and you won't have the problem again. (Hopefully)

I had a tube where the valve didn't seal well with every use. I'd check and fill the tire, and everything's fine. Next week, check and fill, but this time the valve wouldn't seat just right, and the next morning the tire would be nearly flat. Never found a leak, but after a few times I decided to check the stem so I filled it and check... nothing. Let some air out, and filled it and check... nothing. Let some air out and filled it and check... BINGO! There's the problem. Didn't leak every time, but it was a leaker.

Of course, you could go nuts on a completely innocent person and escalate the whole thing into violence, termination, lawyers, incarceration, etc...