A weep or a trickle of water appearing at the front of the machine, worst on a hot wash.
Usually: The door seal has hardened with age and detergent, lost its shape and stopped bearing evenly on the glass — or a fold in it is holding grit that has cut a channel through the sealing face.
On the visit: We run a short cycle with the front panel off to see where the water first appears rather than where it collects, then replace the seal and set it seated correctly all the way round.