as above, an external pic would help?
perhaps moisture is getting under the door sill?
perhaps the door has been installed too far forward - frames should be set back a min of 75mm from the face of the wall.
is that nib a masonry nib or something framed up and boarded?
is that a DPM peeping up at the abutment?
if it is then why does it suddenly stop on the nib - it should run right through under the slab until the cavity wrap up.
as a DPM it should be flopped up behind the plaster or plaster board - not in front of it.
the solid plaster or plasterboard should finish short of the floor by say 40mm to 50mm - skirting covers the gap.