A puddle of liquid on the floor of your home isn't going to set off any alarm bells. You simply wipe it up, and that's the end of it. Those alarm bells might go off when you notice a damp patch on your walls. It could be something minor, or it could be the beginning of a plumbing emergency.
The Process of Elimination
You need to begin the process of elimination. Could something have simply been spilled on the wall, as opposed to a leak that has begun from inside the wall? It might be a spilled drink, a small amount of splashback when watering a plant, or rain that has blown in through an open window. Any of these would be preferable to an internal leak, although chances are you would be able to pinpoint the cause of such an occurrence. The water might also be beading on the surface of a painted wall, since it has been deposited on the surface rather than emanating from within. But what about when you're fairly sure that the water is making its way outwards?
Behind the Damp Patch
There's likely to be a problem with the pipe directly behind the damp patch which has allowed water inside your wall cavity. Have you noticed any changes to your water pressure when you run your taps? Though not conclusive, this can be a sign that you're losing water from somewhere in your plumbing system. You will need to utilise the services of a local plumbing services company. If you're concerned about the potential for damage or accident while waiting for them to arrive, you can disconnect your home's water supply. You might also want to disable the room's electricity supply to prevent the water from affecting any wiring within the wall.
Accessing the Issue
While the plumber will attempt to be as unobtrusive as possible, it's important to remember that they might need to cut into the wall to access and then repair or replace the pipe in question. The incisions will be as small as possible, but the wall will need to be replastered and repainted once the the work has been completed. It might sound like a significant amount of effort in order to locate and repair what is likely to be a small leak, but if left unchecked, the water can wreak havoc inside your walls, damaging circuitry and even beginning to corrode the walls themselves.
Though having your wall cut into to access a leak can feel dramatic, it's far less dramatic than what can happen if the water is allowed to accumulate inside your wall.