I think they've been here for awhile. This is the best pic I could get of a "hole-puncher" that was zipping around behind me on a trip. I have a mode-s transponder, which picks up air traffic around me. It was picking up traffic behind me for a split second and then it would be gone, then it would be back, then it would be gone. Traffic blinking in and out. I was on autopilot looking out the back window. I snapped a shot looking for something my eyes could not see.
[link to image59.webshots.com]

Also, later, I ran into some rain and a huge lenticular cloud, probably about 5 to 10 miles across came at me head on. I dived to stay in VFR weather, and flew right under the belly of the thing. It was weird. Seemed to be moving along with the rainstorm.