About 3 weeks back or so, I was going to PP a target with PP mode before digging, and found my GT making a rather unusual response over the target. Often I PP in disc as I worry about iron dragging me off a coin in PP and me digging a nail, and perhaps forgetting to re-check the area to see if a coin lurked there too.
Anyway, with the odd response when I flipped to PP, I panicked, and thought something was wrong with my beloved GT. Turns out the all metal modes switch was set to all metal fixed mode, instead of PP. I hadn't used all metal for a while and so wasn't used to it's audio response. Man, that one had me real worried until I saw the switch was set wrong. I use a remote PP switch on my GT, so rarely do I touch the face plate switches for disc or the all metal modes.
Never found I bumped the disc or notch dials though, but I do adjust all my pots and switches from time to time, running them through full spans of motion. Prevents oxide build up on things, which in the end can cause something to go bad.
I have servos in planes that I was always told to exercise a few times a year even if I haven't flown the plane. The surface areas the pot feelers touch can get a residue on them from the air and such, but by moving them in full motion once in a while that will prevent such build up.
I've even heard from a few RC guys, that a plane they brought out of storage will have real fluttery controls (pots) until they've worked them back and fourth a bit. Makes sense to me, as I've found pots that wouldn't sit still often need a good cleaning with non-residue electrical cleaner to bring them back to stable life. The servos, when dirty, can wander or will fluctuate back and forth real fast. Imagine what will happen then to a dirty pot on a detector. It can make things go very odd, more so than you would think, if the resistance is changing fast back and forth.