Have you recently moved into an older home and understand that you may have to do a certain amount of renovation? Ideally, you would have had this property inspected before you bought it and so you shouldn't expect to come across any major issues. However, you have noticed signs of a problem in and around the shower and you're wondering what may be wrong. Could this be something significant, or is it a problem that you can fix yourself?
How It Should Be
As you may know, when a shower is installed, it should be accompanied by a waterproof membrane that sits behind the tiles and grouted surface. This membrane will catch any moisture that happens to get through some of these semi-porous surfaces, and if everything has been configured correctly, no problems should arise. However, if you can see some damp patches on the drywall just outside the shower cubicle, notice that the tiles have changed to a darker shade or have to deal with a musty smell on a regular basis, one of several issues could be apparent.
List of Culprits
First and foremost, the aforementioned waterproof membrane may simply be absent. However, the tiles may not have been correctly bedded in or fluid aprons omitted when the taps were installed. You may find that the flange is missing from beneath the drain on the floor, or you could have some leaking pipework behind the walls. It's also possible that you will have pinholes in your grout or that the grout was poorly applied in the beginning.
Addressing the Issues
You can certainly try to address smaller problems yourself and see if that has the desired effect. For example, you could try to repair the grout if any of it seems to be cracked or otherwise missing. If you do this, you should use a special booster in place of water to make it more flexible over the long run. Always use silicone in areas that have to deal with a lot of water (towards the floor, for example) and spray the entire area with grout sealer when you are finished.
On closer inspection, you may notice that the grout itself has definitely seen its better days. There may well be a waterproof membrane, but the amount of moisture that is getting through is causing an issue anyways. To move forward, you should bring in a professional to perform leaking grout shower repairs.