No red line appearing in the hammer
Hello, i am currently working on making a map with the hammer for portal 2, but i am having some leaks, and the big issue is that i don't see any "red line" that people talk about all the time.
Here is what the "run map" console thing says:
Entity func_brush <-28.00 106.00 -152.00>
I did not find what entity it was but i managed to figure out i can just make a giant cube around the whole map and then make it hollow, do that if you have any leaks! Works perfectly!
Uh no, NEVER. do that. That may fix the leaks but it can cause huge performance problems like lag.
Instead, go to Map > Load Pointfile and a red line will point where your leak is. 
To me, it looks like one of your walls is a func_brush. Brush entities don't seal the map.
One possibility if no red line seems to exist is that one of your func_brushes has an origin outside the map. The "origin" of a brush entity can be moved anywhere you want, potentially far out in the void, and the red line would lead from that point to the void near it.
Try using "Go To..." to go to the coordinates (-28.00, 106.00, -152.00) as reported in the console, and see what you find.
Hi !
If "go to coordinate" as say FelixGriffin doesn't work,
my personal method is to do what you say with your "giant cube nodraw" around your map, it will confirm that your leak is due to a "hole in your map with the outside", but then you must tick "Don't run after-compiling" and you narrowed the area to nodraw progressively to target where there is a problem and fix it.
As say RectorRocks , you can not let this cube nodraw: you will have big problems
Most often it happens:
--Either because you've converted brush (or transparent texture) a func detail on the edge of your map unintentionally
--Either because you used the fine grid for arranging your blocks and there is a lag of a few millimeters that makes a hole
--Either because you used a func_detail as thin as possible, and that is not properly adjusted
Hope this help you
...You did load the Pointfile, right?
If you did, and there is, for some reason, still no red line, make a TEMPORARY hallowed-out skybox cube around your map (remove it once you're done). If it compiles with no leak, you have a hole in your world. If it still compiles with a leak, you might have an entity outside of the world. Remove the big skybox and fix it.
I know I used to accidentally drag the little sphere thing on entities like brushes outside of the world, causing leaks.
I am so sorry, i've been very inactive here, thank you for all these responds. They were all very helpful! 
I also often get this bug. It's really annoying to notice that's there's apparently a leak in the map but the pointfile does not show where it is. Sometimes it does show a red line but it points from nothing to nothing. Yes, a red line floating in the middle of nowhere. I tried to fix this error by checking the brush work, the entities' origins... in vain.
However, fixing errors such as : entity <entity_name_here> has unused keyvalue "angles" or "maxdxlevel" in the Check for Problems window (Alt + P) fixed the problem.
I know that fixing errors listed in that window is something we should always do from the first place, but I don't see how unused keyvalues can cause leaks in a map lol. Weird...
However, fixing errors such as : entity <entity_name_here> has unused keyvalue "angles" or "maxdxlevel" in the Check for Problems window (Alt + P) fixed the problem.
I know that fixing errors listed in that window is something we should always do from the first place, but I don't see how unused keyvalues can cause leaks in a map lol. Weird...
The question's already been answered a while ago... ;-;
However, fixing errors such as : entity <entity_name_here> has unused keyvalue "angles" or "maxdxlevel" in the Check for Problems window (Alt + P) fixed the problem.
I know that fixing errors listed in that window is something we should always do from the first place, but I don't see how unused keyvalues can cause leaks in a map lol. Weird...
The question's already been answered a while ago... ;-;
I was just trying to help... Sorry if my post feels inappropriate :S