Imagine situation ...
You send couple of Goblin Scouts to search for a Grove ... they find it ... they are all killed (or captured in Sazza case) ... therefore you send another group ...
>> There was attacks recently ... you still dont know where it is.
Another mystery solved. :3
And you see that where exactly?
You see, you don't really know. You're just guessing. And that's my point. There are many details that are left for the player to just guess what they're talking about.
And based on your assumption, what you are suggesting would mean that the goblins were wiped out every time they found the Grove during every attack. Not one goblin was able to escape. So the tieflings have fought off the goblins from the top of the wall several times now, and not one goblin was able to get away. Is that what you're saying?