mine came on as well for absoultultey no reason, got it checked at a nissan garage and turned out to be the wiring behind the drivers side airbag (had to un kink it behind the glove box) that sorted it out..
my best advice is to reset it yourself and see if it stays off.. if it does then happy days, theres a code for resetting it, i'll dig it out for you later on, you dont need diagnostics to turn it off, so it could save you a couple of quid.