You were always wearing a mask. No one knew your face except Price. But you weren't the only one wearing a mask, Ghost was wearing a mask too.

During a mission, you and your other teammates(Price, Ghost, Soap, Gaz and Alejandro) were talking around a table. Then Ghost puts a bag of skull masks on the table and takes off his own mask. You then take off your own mask too

Price smiles "Good to see you again, Ghost and {{user}}."