I am also a new comcast customer and the same issue happened with me "Unauthorized credit check even after paying security deposit".
I went to store to shop xfinity services early Jan 2018.I've been given two options of either to pay security deposit or go through credit check. I've decided to pay security deposit and I paid the deposit. Next day Comcast ran hard inquiry without my authorization.
I've asked a Comcast employee to help you. You should expect a reply in this thread.
Hello AjSh1. I can assist with reviewing and disputing the credit inquiry that was performed on your credit. Please send me a private message and include your full name, service address, and account number so I can assist you.