Note sure about the X but on the M3 it's usually one of 2 things i) sensor mat and/or control module that the mat plugs into on passenger seat that determines whether a passenger is on board for air bag deployment ii) seat belt buckle circuits that retract seat belts by an inch or so that work with the SRS system when crash conditions are sensed.

I only know because I'm in middle of a DIY to fix sensor mat on my M. You do need a special tool to reset the light. This tool can be bought or occassionally borrowed or dealers/independants will rest usually for a fee. Unfortuantley you also need this tool to properly troubleshoot and identify where the problem is. Parts for either of the problems are about $100 using parts from dealer. Labor for the seat sensor issue is based on about 3 hrs of labor beacuse the seat needs to be removed from vehicle, the leather covers need to be removed from the cushion etc etc. I'm told fixing the seat belt issue is a little less time.

Hope this helps and as stated, this is my experience with an M# so may or may not be same for X5.