It just happens when you take your seats out. If the battery is not unplugged for at least 30, preferably 45 minutes before you remove the seats, you'll get the airbag light. You'll also get it if you don't reconnect the electrical underneath before reconnecting the battery.
It won't go out until you go to the dealer and get them to clear it. They'll probably charge you close to $100 to clear it unless you can find a local P-car mechanic who can do it.