Coolant low message

I got my car towed one day because my tire was flat and the tow truck driver scraped the bottom right of the car pulling a panel, I got the panel put back and fixed and got a coolant low message. I checked and the coolant was in the middle of the max and min mark so I filled it up just to the max and it went away, a couple days later it came up again as I was going up and hill. So I pulled over looked and the coolant was still at max, I ended up replacing the sensor with one from bmw themselves and now it comes on everytime I start the car, is this a faulty sensor or is it more than I think? Its a 2012 528ix