can the mechanical brake pad wear sensor be tampered with by a mechanic

I'm sure the front pads are worn down. The cars has 70K on it and I've never had the brakes serviced. I had the car in last week to buy a new set of tires at a deep discount due to a serious error of omission (not tightening the lugs after a tire rotation on my daughters car). Well less then a week later The wear sensor is sounding off loudly and unrelated the emissions control system check light is illuminated. Seems like too big a coincidence. I think they want to get back in the black by doing additional full retail work based on the two alleged issues

Well the sensor on the brake pad is just a metal tab stinking out, as the pad wears eventually the metal tab will start to make contact with the rotor and that's the " loud squeal" you hear, letting you know it's time to change it. Brakes should be check more regularly, if you catch them in time you can save your rotor from getting too much damaged. As to the check engine light, get it scanned at any parts store and post the code here, so we can help guide you. Could be something simple or more in depth, won't know until you get the code. By the way if they forgot to tighten lug nuts I wouldn't go back there anymore, that a careless and dangerous mistake.