still no definative answer.
i check for errors regularly. there is no instance of any empty entities (something i've not seen for a very long time, btw), or any other errors. anyway, that would put the entity at 0,0,0 (the grid origin) and not 0,100,4
here's another pick. i used edit>select all, to highlight everything, instead of the select tool. this would highlight even empty solids, but as you can see, there is nothing highlighted at the co-ords stated. the entire level is well below that area.
the evil carve tool has never been out of it's box, so i can't point the finger at it. all of my rockwork is done by adding edges to simple blocks, and moving the vertex points, so it's not like i've clipped a brush and ended up with floating points either. this is one of the reasons i use alt+p regularly. to make sure i've not made an invalid shapes.
this is definately more akin to func_water drowning you in parts of a level where it doesn't exist. some of you (if not most) will have seen it at some time. seems to happen when func_water is either a complex shape, or is next to one. when it happens though, there wont be any leak created if it extends into the void, as func_water can exist there anyway.
this makes me wonder how many times this has been the cause for the threads that go something like, 'whats wit dis f00kin leak s**t - my maps is teh perphakt'.
the answer would probably be none, but i wonder about some strange stuff at times. 
.........................................................
wow, another strange thing just happened.
in preparation to offer up my bsp to thomas, i retextured the whole map (other than sky and hint) with a generic hl texture, so that he wouldn't have to dl the wads for it too. i lowered the top of the sky back down to below the co-ords to expose my phantom func_wall to the void again, and ran the compile proccess to confirm the leak was still happening. it was. but the other strange thing was in the log. i had exceded max clipnodes. seems very odd that retexturing something would increase the clipnodes. other than maybe free-standing water, which i dont use much of at all.
I'll go hide in the corner now.
[addsig]
hate is bad. very self destructive. so no, i wont hate you. i might throw things at you or kick your dog or somesuch, only just as stress relief you see, never as hate. 
) 
Gollum?...where are ya???....