Two questions:
I played BG like a mad man when it first came out (back when AOL still had a game board and Baldur's Gate and CIVII were the last active boards). I used Imoen as a scout. I recently started the BG version which you guys call Vanilla and I noticed something I don't remember. When Imoen scouts an area, if I fail to keep the rest of the Party close by--- when they do move up--- they are often attacked. The game infills with enemies. Not all the time, but often. If I do keep Imoen and the Party close together, if I back track, again the cleared area will often be filled with enemies. I don't remember this when I originally played. But that was a long time ago. (I did try the newer version a couple years back but couldn't get use to it.) Did the game always infill with enemies? I don't mind it. Actually it's cool. But I just don't remember it happening. Did it?

Second, before I go much further, does the Big World mod deserve a look? Is it a stable, doesn't break the game etc.

Thanks
Bob

No it didn't and doesn't, that is if you are talking about unmodded, non-enhanced version.

A bug then. But a fun bug.

Thanks

You are talking about the original Baldur's gate?

If so, there are several areas that will backfill with monsters. One example is the fort with the hobgoblins when you got to save Minsc's mage. If you send her forward the area between can get more hobgoblins.

I know certain areas are like that. The Siren/Lighthouse Map has Hobgobblins showing up again and again. No, these are monsters like wolves, orges, etc that aren't there when Imoen (Shadowed) goes through but are when the remaining team does. I actually like it because it forces me to use Imoen more carefully.

Those are not 'backfill', those are just higher respawn rates. The game does not 'backfill', ie. immediately respawn, in any area.