Ok well, right after the brahmin where you save the tribal hostages there is a small outpost with one raider. I tried opening the door, I tried lockpicking the door, looked for a key and couldn't find anything to open the door. If you still don't know what im talking about, I'll try to find a pic. it's from the very first mission in Brahmin Wood in Fallout:Tactics released in 2001.
I know the place... Its a guard booth surrounded by barricades right?
I'll see if I have a save game there... As I recall I couldn't open it either ~and then suddenly I could.
I passed it and moved on,but I don't recall how (that doesn't help you at all I know... But I'll look at it again, if I have a save near there). ~If not... I suspect you have to complete some thing (if not find the key somewhere).
EDIT:
**I don't have one near there.
My best memories of it were there was a dead soldier in there and I walked around the barricades a lot before getting in... I don't recall if its a key or a switch... Someone else will have to post about it.