They should be prop_static.
Leaks are gaps in your map that 'leak' into the void beyond. Imgine you only have enough air for your map and your players need to breath it, if there is a hole in your map all the air will escape, and thats not good, you've got to fix it but plugging the hole.
go to: map>load pointfile
It should give you a red line, follow the red line and it will pass though a gap. Plug that gap, recompile and look for the next leak. (the pointfile only shows the first leak it finds and all need to be fixed.