When I first went diving in colder water, I was coming up with some blood in my mask, which I only noticed when others mentioned it. No apparent sinus problems at the time, for all I know I do it in warm water as well but as I tend to remove my mask in the water then, it just might not be noticeable.
It didn't worry me much, likely wouldn't have even if I hadn't known it was common when diving, but it seemed to freak out a few of the divemasters-in-training that were there at the time...
If there's a little blood in the nose of my mask, it's probably nothing to worry about.
If, on the other hand, I'm hanging around on dry land having not been anywhere near water recently, and my nose just starts spurting blood at random, getting drops of blood on stuff and taking about 10 minutes and a large pile of kleenex to get under control, then you might consider worrying.
Even if it's *still* no big deal, it's a bit more polite than being completely oblivious and after it's over and I've dumped the pile of kleenex, denying it ever happened and saying I was just making it up to get attention, even though you were sitting across the table from me the entire time (on your cell phone, of course). That's just rude.
Mask boxes are halfway decent for storing tissues on a boat, in the absence of any sort of dry bag.
