Help please. Damp patch

I've just looked under a mat which lays onto of a lounge carpet and found a large damp patch. I can't work out where this water damage has came from. It's a chip board floor and it's a ground floor room within a semi detached house. I don't know how to go about checking where there is a leak? Any advise really appreciated.

Make sure it is from under the floor rather than above. Any pets, does it smell? If no one owns up to it, remove carpet and see if the wood is wetter than the carpet. If it is the you need to find the joists and cut the board on the joist line to access the leak underneath which maybe a pipe with a hole in it spraying up onto the board. BUT check for all possibilities above the floor first. If cutting boards to access there may be a pipe near the board so DON'T cut too deep.