I think there are 3 possibilities. One is tear from the eye, which drains into the inferior meatus of the nose. Two, is normal mucosal drainage of the nostril or sinuses. And three, is water from the sea or lake.
If your mask is relatively dry, it might be possible that air flow from the regulator can blast droplets up the pharynx and they might settle in one of the superior sinuses.
I am guessing that it might be tear. The normal drainage is down the nostril and out the nose. The diving position is often the superman position. If one were diving with a heads down and legs up - it is possible for the tear to flow cranially or superiorly into one of the superior sinuses (
http://www.american-rhinologic.org/patientinfo.sinusnasalanatomy.phtml). It also can possibly jump into the middle meatus of the nose, and drain into the maxillary sinus. Perhaps the tear might even be trapped in the tear duct and remained trapped due to positive pressure on its opening.
It is often quoted that the sinuses produce about 1 quart of fluid a day. A 30 minute dive can produce quite a bit of fluid from the sinuses. If their normal drainage pattern is disrupted, perhaps they can accumulate in sinus cavities. They are excreted by ciliary motion of the mucous lining and by gravity. Perhaps the superman position cause too much fluid to accumulate beyond the capability of the ciliary elevator can move - so when you bend over - they are dumped out. We should ask superman if he has this problem.
There might be a way to find out where the fluid. Apply some flouresceine stain to both eyes before you dive the next time. If the snot that comes out glow green it is likely tear. If it is not, then it might be salt water, or sinus fluid. Of course, if they all mix together .... we'll never know.
