not being one to want to tamper with things, but......
given the slight fade or difference in texture of the paint between the helmet as a whole and the area where the decal is (was), doesnt it look like the decal is still there????
there is a slight lip/ ledge to the decal outlining it, or at least where it may have been.
I would be tempted to try and lift just a millimeter of paint from the area to see if anything is underneath.....
but that depends on how much i was into the helmet for....
