Had the same thing happen...long story short, get your codes read, have them reset, see if it comes back.
SRS system consists of airbags, seatbelt tensioners, a sensor that determines if the passenger seat is occupied or not (!), the seatbelt sensors that BImmerN2B mentions, all kinds of spookey expensive stuff.
My code reading said the passenger seatbelt sensor was the culprit. I reset the code and it hasn't come back (six months now).