A few things to check or do:
1) Compile again - the red line might appear drawn differently and thus reveal the actual leak location.
2) Check there aren't any stray entities clearly in the void, sometimes the red line is just drawn strangely.
3) Check the wall is not a detail brush (or brush entity) by accident.
4) Check that no brush entity or patch mesh resides partially inside the wall brush (this might be a hard one to catch if you don't pay attention).
5) Delete the wall brush and draw it anew.
Frozen by ICARUS