It's very obvious that there was a leak, but I'm not sure if there's still one. In any case, I'll be checking pressures at least once a day.
After reading the replies, I'm thinking maybe a grain of dirt could've gotten into the valve stem and caused the leak. Later it got dislodged, and that's why it appears to be fine now.
If it still leaks, I can use my bath tub for diagnosis.
As for preventing this in the future, since I need to get some new tires, I'm thinking about getting an aftermarket TPMS. Anyone had an experience with an aftermarket system such as this: »www.amazon.com/Orange-Electronic ··· 02DYH586--
Wacky Races 2012!