Well, my car is apparently OK! The tire did not actually have a leak, which I thought was odd, given that my car dropped from 31 psi (after I filled it up last night) to 25 psi overnight. Only two scenarios I can think of: tire gauge is defective, or "someone" (*cough* illegal resident *cough* ) may have been messing with the car.
I'll get another tire gauge, and if the same problem persists, I'll go for a second opinion, I guess.