There should be 1 sensor on the front and 1 on the back, I cant remember which side these are on. As has been mentioned before, if the pads are OK then start to suspect the instrument panel, when these are faulty they can flag up incorrect warnings. When I bought my car the dealer had hidden the instrument panel fault by adjusting the bonnet closing switch so it didnt switch when the bonnet was closed, this showed up as the indicators didnt flash when I locked the car. When I adjusted teh switch the brake pad warning showed up! Thanks to members of this site, I was alerted to the problem after around 6 months of owning the car, my fight with the dealer to get it resolved is somewhere on this site.