If the pads are old and compressed, the velcro that holds them into your helmet can push into your skin leaving long lasting marks. Replacing the pads might solve your problem.
I rarely ride without a band under my helmet. A sweat band during spring, summer, and fall to keep sweat from dripping in my eyes, and an ear warmer in the winter. They both help keep the helmet pads from making any marks my forehead at all.