I have responded to a lot of calls to look at leaky showers over the past 13 years. Almost all of the showers that I have inspected have leaked at the door or metal track that holds the glass. The leak itself is not always noticeable, but the damage it causes over the years is obvious.
The result is rotted baseboards, water damaged Sheetrock, and mold where the shower meets the floor just outside the glass door. The good news is that when faced with signs of water damage isolated to outside the shower door, almost all leaks have been the result of an original installation error. The error is in the way the shower trim was sealed with silicon caulk. The mistake is compounded over the years by handymen and DIY'ers adding more caulk to the inside of the shower as a repair or part of home maintenance. Don't worry, it is a common impulse to keep adding caulk to the shower in anticipation that the leak will eventually stop. Not only is this installation error common, I have witnessed it in 100% of showers with the same water damage pattern.
Corner Metal Shelves
The most important lesson when sealing with caulk is "less is more." As it is very important to thoroughly caulk the outside of the metal shower trim, it is equally important not to caulk over the "weep holes" inside the shower. These are little drain holes in the bottom of the metal track inside the shower and on pre-made acrylic shower pans. These holes allow the condensation and water inside the metal track to drain back inside the shower instead of being trapped and eventually finding an alternative escape route. For the system to drain properly, I also suggest not caulking inside the track at the corners. The faster the water can flow down the track and out to the shower drain, the quicker the shower will dry. This reduces the chance of mildew and mold growth tremendously.
The type of caulk you use is also key to a long lasting sealed shower trim. 100% silicone caulk is the only caulk that has stood the test of time for me. It doesn't shrink or crack unless it is in direct sunlight, and it stays flexible for a long time. Acrylic caulk hardens, cracks, and is less successful in keeping out mold and mildew. As a rule of thumb, it's easier to match clear silicone to the metal trim and plumbing fixtures while white or grout color matched silicone is easier to match the corners of walls, shelves, and benches inside the shower.
Last but not least, check to see if all of the rubber seals are still in place and in good shape around the glass, the sides of the shower door, and the sweep at the bottom of the shower door. These are there to help shed water down the inside of the shower and help keep splashing through any gaps in the door to a minimum. These items can be inexpensively replaced at a glass supply shop.