That, or maybe the adjacent piece of track has a rail just a tad too long and is contacting the sensor rail. Or check and make sure the copper J hook on the adjacent rail isn't bent slightly out of shape and contacting the sensor rail.
I have MTH RealTrax on my current layout (purchasing both new track and used track) and I noticed that the rail length tolerances were a little loose on all the track pieces. Nothing consistent, I just had to be careful with any piece of track attached to a switch.