I re investigated the issue. There was a actually a little dot on the ceiling as well between the two lines. (which might not have been there last night...or I missed it).

Wife tapped the ceiling while I was in the attic. Under the insulation it was right next to the landing for the a/c unit (opposite side of the drain pipe). I found maybe a flake of wet insulation while pawing through it, but the wallboard was a little damp. This is about even with where the roof vent is (center of the house). The lines run perpendicular to the joists. I think water hit the wall board at a point (the dot) and ran to the edges along the joints where the tape is.

I still don't know for certain what caused it, but I'm pretty sure it's not a hornet's nest. If there was a 4 foot hornet's nest in my attic, I would just move. :lol:

Roofers come Saturday morning. I guess I'll just have to break out the paint.

Prime the stains with Killz before painting to avoid the stain bleeding through.

You may want to put a tarp in the attic around the area of the moist flakes to see if any further wetness is generated when there is a storm or when the AC is used. The tarp may also prevent your paint job from becoming a recurring chore while your determine the cause.

^x2 on the Killz. I tried several coats of regular paint when I was spotting and stain eventually bled through. The Killz primer works well but PU! Ventilate - should be good weekend for that. Hope its simply resolved.