Issue with TPMS and Review of Pirellis

[JonX]

K ive had the stock 17s 235 55 17 pirelli scorpions...1 tire had a slow leak coming from an old plug...had it double plugged, still slow leak, decided to get new tires. Went with Pirelli Pzero Nero 235 55 17 right off the bat, these tires are softer (ride is less bumpy) and quieter (granted the tires i replaced had 45k miles and was wearing okay. Now my issue with the tpms is the light has come on obviously from the 1 slow leak, filled up a couple of times and resetted with the button on the center console. Today i changed the tires at work...to my suprise NO sensors in the wheels?! So of course on my way home from work, tpms light comes on, i recheck tire pressures everything is at my set psi...hold the button to reset tmps and light still stays on. My question is where exactly are the sensors??