You have a leak. Usually it is the gulley that is broken and is leaking into the ground, but in your case the mark extends above the DPC and may be due to an overflow, splashing, or a leak higher up from the pipe running into it.
1) is your wall built with lime mortar? I believe it is Lime
2) does the gulley fill or overflow? Not that I have seen, the previous owner did have drain works done a few years ago.
3) is the floor of the room on the inside of the mark concrete or other solid material, or is it wood with a ventilated void beneath? wood with ventilated void
4) Does a washing machine discharge into the drain, the gully or the downpipe? The staining is typical of white soap deposits from a leaking washing machine waste pipe. No this is a combined rainwater from the gutters and drains from the upper bathroom and lower bathrooms. there is quote a large hopper where multiple pipes discharge into above this photo, so maybe in the past this was overflowing or full?
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