Not my Glocks. Sorry, I just had to say that. But you are correct about that. I once had a Series 80 Colt Government Model 1911A1, that was made of stainless steel and all I had to do was to carry it in a holster while deer hunting (during heavy fog) and by the time I got home, light rust was already present. An old machinist once told me that stainless-steel means exactly what it says, stain-less, not stain-proof.
LOL! That's because a glock is mostly made of a high-strength nylon-based polymer. Also what metal there is has Tenifer treatment which protects the metal even if the decorative surface finish wears off. It's almost a true plastic gun. My Taurus Millennium Pro PT745 is similar in design material.