For stains of an unknown origin, try following these steps: First, rinse the stain in cold water. Pretreat it with a prewash, then rinse again. Now wash the garment with an all fabric bleach, in water that's as hot as the manufacturer recommends, adding extra detergent. Let it are dry and if the stain is still there, soak in cold water for 1/2 hour. Sponge it with a cleaning solvent, let it sit for 5 minutes and then rinse again. Repeat all of the above if the stain persists.
As silly as it sounds, baby wipes are a great way of removing stains and are also great for cleaning in general. They pretty much can remove anything!