Light getting into a camera where it should not, fogging one side of the frame with a warm coloured glow.
Under it is a scattered field of six slowly drifting points, each carrying its own colour pulled most of the way toward the tint you chose, so the leak reads as one hue with variation living inside it.
Left there it would look like flat polygons, so the field is smeared. The brightness of each patch is read as a direction, the picture is pushed along that direction, and the field is evaluated again from the new position. That second pass is what turns blocky cells into the streaked, drawn-out shapes a real leak makes.
The glow falls off from each point, and the whole thing is screened onto the picture so it can only ever brighten. A mask holds it to the edges of the frame, which is where light actually gets in.