Probably a slow leak from something embedded in the tread. The fleabay part is not the complete TPMS unit and you won't need one unless a troglodyte damages it while dismounting your tire. If it's a run-flat then be sure the shop has experience as the tire bead is really hard to shift. Putting a patch on the inside is the safest repair but the tire's maximum speed rating is reduced - not a problem in most countries!