red floor tiles damp patch

semi house built around 1910
we have 1 ground floor room with red floor tiles built direct on the ground. there is a small corner of the room (2 x 4) tiles approx that has what looks like damp coming thru.
there are no water pipes or other services in this corner and house next door has no issue.
from work we had done on the outside opposite wall it looks like the room is built on sandstone.
any ideas?

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