Hello Keith, welcome to the forum
I'm not familiar with your brand of detector, but the detectors I am experienced with
( Nautilis) do have a Red button on the end of the handle. Conventiently placed for often adjustment. It is used for keeping the threshold ( Hearing level ) tuned to a steady comfortable hearing level.
The drift level of the detector will either increase or decrease in volume caused by warmup, temperture changes, or high humidity. And also by little bumps of the coil on ground or other objects. My machines after they are ground balanced properly do a good job, but have to have the red button depressed often in order to keep the volume at a steady & comfortable operational
level. If the red button on your machine is located in a convientent place on the handle then I would assume it is there for that purpose. Without first hand knowledge of your type machine I cannot definiately say that is it's purpose, but is very possible. Glad to have you with us, keep us informed of your finds.
HH
SS