Work out when the water appears
This single observation narrows the cause faster than anything else. Run the shower with the door closed and watch. If water only shows while the shower is running and stops soon after, the escape route is somewhere the water reaches under spray: the silicone line, the grout, or the tray to wall junction. If water keeps appearing for a long time after the shower is off, suspect the waste or the trap under the tray, because the tray is still slowly draining into the void.
If water appears when the shower has not been used at all, the leak is a supply pipe or the valve behind the wall, and that is the one that needs someone out quickly. Pressurised leaks do not get better and they soak structure rather than just wetting it.
Mark the damp patch with a pencil line at its edge before you test. If the line is still the boundary the next morning, the leak is intermittent and linked to use. If it has grown overnight, it is a supply leak.
The silicone joint, the most common culprit
The joint between the tray lip and the tiles takes constant movement. Every time you stand in the tray it flexes a fraction, and cheap silicone or silicone applied over old silicone will split along that line. The split is often hairline and invisible until you press it.
To test it, dry the joint completely, run your fingernail along it and look for lifting, blackening or a section that has come away from the tile. Anywhere it has released, water is running behind. Re-sealing properly means cutting all the old material out with a sharp blade, cleaning the surfaces with methylated spirit, letting them dry, then applying one continuous bead of sanitary grade silicone with the tray filled with water so it is loaded in its worst position. Do not skim a new bead over the old one. It will fail in weeks.
Waste, trap and the joint you cannot see
Shower wastes fail in two places: the rubber seal that clamps the waste body to the tray, and the compression joint where the trap meets the pipe. Both are cheap parts and both are behind the tray. If you have a downstairs ceiling below with a hatch, or a bath panel on the other side of the wall, it is sometimes possible to see the drip forming.
If you can reach it, dry everything, put kitchen roll under the joints, then pour a jug of water slowly down the waste. Wet paper tells you exactly which joint has gone. It is a far better test than guessing.
When the waterproofing has failed
The serious version is water passing through the grout, into the wall board, and down inside the wall. On showers fitted before waterproof boards and tanking became normal practice, the tiles were often stuck straight onto plasterboard. Once the grout crazes, the board turns to porridge quietly for years.
The tell is tiles that sound hollow low down, grout that stays dark, or a skirting board on the other side of the wall that has swollen. At that point re-sealing is a waste of money. The wall needs stripping back, boarding in a waterproof board, tanking and re-tiling, and the tray usually comes out to do it properly.
What we do on a leak visit
We start with the cheap causes, because most leaks are cheap causes. We check the silicone, the grout, the waste and the valve, in that order, and we tell you which one it is before doing any work. If it turns out to be the seal, you get the seal. If it turns out the waterproofing has failed, we will say so plainly and quote for putting it right rather than papering over it.
If you want it looked at, send a photo of the damp patch and roughly how old the shower is through the enquiry form and we will tell you what we think before anyone visits.