Sweat causing rust. My Bersa Thunder will have a few specks of surface rust on the left side of the barrel after a day of carrying IWB on my right hip. Wipes off pretty easy. Also the SW Airweight I keep in my pocket had the screw in the grip rust over pretty solid. I keep meaning to go get a stainless screw at the hardware store.
One other thing I didn't expect was how easy it would be to carry every day. Find the method that conceals best for your daily activity, and you'll never wake up and say 'Man, I don't feel like carrying today.' It's just a habit now.
Rock out with your Glock out!