Was driving along when the TPMS light came on, pulled over and checked the tires to make sure I didn't have a flat. Tires felt OK so I kept driving the last mile home. Checked tire pressure two days after and all tires were 3 psi under. Proceeded to inflate all tires to 33 psi, light didn't go out. Turns out this is a cheap tpms system where you don't have a sensor on each wheel but instead the ECU monitors differences in wheel rotation and compares between each tire, so manual says to re-calibrate each time I add air by pressing this little button. Well I press the little button and wait for it to blink. It doesn't blink. I try it with the engine on/off, door open/closed, idling/driving down the road, nothing makes the thing re-calibrate. Have to take it to the dealer now, I read online TSB 14-006 is supposed to fix this, so we'll see.

Update from Feb 18, 2016: Took car to the dealer. After inspection it was determined that the harness connection to the TPMS module was disconnected. A couple of months ago I took the car in to have a remote engine start installed, which required a large amount of work under the dash. Likely it was just a connector that didn't get re-plugged in properly.