I suspect most General Aviation pilots (who fly mainly for recreation or a hobby) wouldn't know what it meant, although I'm pretty sure we're supposed to. Assuming the pilot knew what it meant, they would most likely make a note of the position and radio in a distress call to whatever Air Traffic Control entity they were in communication with. If unable to communicate directly with ATC, I would feel duty bound to report it as soon as possible, even if it meant diverting my flight to a different location (e.g. returning to my point of departure rather than continuing on to my secluded cabin by the lake).

I'm inclined to think that 3 fires in a triangle on the ground are unlikely to be noticed by a pilot in the air unless (a) the fires are accompanied by a lot of smoke (burn lots of green, slightly damp material) or (b) the pilots are specifically looking for a downed aircraft or missing person(s). But, as Chris Kavanaugh's experience (documented elsewhere on this website) shows, some commercial pilots are heads-up enough to spot it and realize what it means even if there are no missing persons reported.

After all, you don't need "most" pilots to report it, you just need one. <img src="/images/graemlins/laugh.gif" alt="" />