I have had a phenomenon going on with most my basses this year. If they stay in the case they'll stay at pitch for weeks, even if taken out for a rehearsal and put back in. But now that I have them stored on wall hangers I notice more and more that the strings go sharp (usually just a few cents) after a few days. What does this mean exactly? Are the necks flexing, is something pinching the strings in the nut, what? In the past strings would go flat, not sharp, if anything at all. I don't think my Rickenbacker 4001 does this but the others that have conventional truss rods all do. Maybe that's a clue. Just wondering if this is a diagnostic for anything to watch out for like over-drying or something. I expect it's just standard weather-wear, but I'd like to know if it's not.