It actually works! Amazing, I know. I was driving down the freeway when al of a sudden I hear the same beep as "low gas" and the tire pressure light was on.
So I'm trying to think of when I could've possibly ran over a nail or something. I stop at the nearest gas station and it turns out that there's no leak that I can tell, but the right rear tire was about 10psi low so I go to refilli it and there's a red rubber washer type thing under the cap that was pressing against the stem and causing it to leak very slightly.

Has this happened to anyone else?