As said there was a leak on 1 side that shifted the internal valve but if you've fixed the leak or there was no leak & it shifted over a bit by itself or if it was slightly off center a bit from day 1 & it finally shifted over enough to make (electrical) contact you are still safe as it is there to tell the average joe that one side has a leak and the center valve shifts toward the side w the lower pressure till it (electrically) makes contact and if there is no leak now you are fine. Hard to tell which way it needs to b shifted but I'd crack either the front half (at one of the calipers so you dont introduce air) or ditto on the rear then hit the pedal to shift it over and may have to try both as you're guessing as to which side but if you had the rears open prior to this I'd now open the front and vice versa.