Is Your Shower Discoloration a Sign of Hidden Problems?

Hey folks, I need your expertise in cracking the case of my shower dilemma. I reside in a swanky 2013 high-rise condo and my shower is in the spotlight here. Picture this: ivory travertine tiles, a sleek glass door, probably a fiberglass pan, and an exposed shower system featuring the Grohe rain head plus handheld combo. Now, here’s the scoop so far: When I first moved in during the Spring, I clocked some funky discoloration on a tile just beyond the shower’s threshold. I got the plumbers on deck to re-silicone the pan, but I’m not entirely convinced they aced the job since it’s already looking a tad worse for wear. They also did a sweep for cracks in the pan, but nada. Fast forward to November, and I spy more discoloration, this time on the bottom row of tiles inside the shower. It might have been brewing for a while, but it didn’t catch my eye until then. Since then, I’ve given the shower a rain check (pun intended) and unleashed a dehumidifier. The interior discoloration has lightened up a bit, but it’s still there, lurking. Now, what’s cooking inside my shower? Could it be the silicone throwing in the towel, water playing hide and seek behind the tiles, or a hiccup with the fixtures? Any DIY tests I can conduct? And who’s the MVP to call in for this conundrum – a plumber, or a tile/stone wizard? I’m all ears for any pearls of wisdom or insights you might have. Thanks a ton in advance for your help!