tbw I forgot to say that I checked every spot for units to be there and one or two even were corpse traps

(btw the last screenshot the huge jump to the left after the first jump is an ulti tower btw I just noticed you don't see that =O)
The only strange thing I noticed is that as soon as I streaked over the point in the first picture always a little dot appeared and disappeared shortly after I finished streak... but I doubt that'll do nothing but confuse you
and it's not 100% that it's caused by dark altar. Dark altar is just a skill that might be worth looking at =P
Edit: Just took this screen in a game with Defiler,Neph,Garg and Firelord
sooo no it doesn't seem to be (only? still dunno if it does but don't think so =P) dark altar
Edit2: but it's probably from the ud because once I had this when playing against elves and now against creeps =o
just a thought... corpse trap leftovers?