Let's please put that to rest.PADI teaches you that mask on forehead is a sign of panic. Other agencies teach you to not panic.
PADI teaches that equipment rejection is one sign of panic. That can especially include the regulator and the mask. A panicked diver may shove the mask off of the face, but in that case, the diver will also display other signs of panic. You are supposed to look at the entire situation to conclude that a diver is in panic. A diver casually climbing the boat ladder with the mask on the forehead is not in panic, and PADI does not say such a diver should be considered to be in a panic.
Unfortunately, some people do teach that, but that is the individual repeating something he or she heard somewhere else along the line. PADI as an agency does not teach that. IMO, it all stems from the unfortunate wording on the old final exam, in which mask on the forehead was the correct choice for signs of panic on a multiple choice question. Instructors preparing students for that exam probably stressed that in their final reviews.