the gear sign on the dashboard

What is your question?

a gear sign showed on the dashboard a year ago but when I turn off the car and put it on again it goes. I took it to the transmission specialist they drove the car and he told me my transmission is perfect. but when the gear sign showed I kept it and when he scanned it said the transmission is slipping and needs to be changed. but when I drive the car I even go top speed and nothing wrong in the shifting. only when the sign is on the car drive slow until I turn it off and on. kindly advice if its a sensor or something else. because its been a year and still the car shift perfect. thank you in advance