This is an interesting problem we encountered on a recent inspection in Aurora Colorado. The area around this drain pipe which services the drain on the clothes washer in this home indicates that leaking has occurred for some time. When I tested the subfloor with a moisture meter, it did not indicate the OSB board was wet at the time of the inspection, but it is clear there is a moisture problem of some kind. In the inspection report, we requested the client have a plumber investigate further.
As the photos indicate, no leaking is visible around the drain in the laundry area. What the plumber determined was that on some occasions, during the wash cycle, the washing machine sets up a vibration which has worked a joint in the drain line loose. Small amounts of water flowing through the drain leak intermittently causing the staining to occur, but it is a small amount and dries quickly. This situation points to the fact that it is always a good idea to periodically check drain lines when toilets, sinks, dishwashers or other systems are in use to make sure there is no leak occurring. Sometimes the seepage can be very small and not readily noticeable.