I too have worked on hundreds of brake system and when I see one front caliper with pads at 5-6mill while the other side is at 2mill and then I glance at the brake fluid reservoir and see dark, green, and generally nasty brake fluid in there, I know EXACTLY why one caliper is sticking and why the owner now needs new brakes and possibly even a caliper plus a fluid flush.
Easily avoidable with a simple brake flush every few years or when new brakes are installed. Most shops don’t touch the flush and just push that nasty old fluid in the calipers BACK UP into the ABS system and call it done.
Surprisingly, that 3-4% moisture is enough to cause me to re-adjust my muscle memory on my clutch leg after flushing my brake fluid. New fluid now compresses differently and I need to relearn the clutch pedal because it now performs differently…. And I flush my brake fluid every few years on this car because it shares the brake reservoir with the clutch pedal.