OK, here's the thing.
I tested it and it seems that even if the bomb is hidden, as long as most of the bomb is shown the bot would probably be able to locate and pick it up. But there were few places in de_dust2 that I was able to hid the bomb deep enough in the wall so most of it wasn't shown and the bot could
NOT locate and pick it up.
As you can see in the pictures, the bot was not able to find the bomb in picture "HalfLife03", "HalfLife04" and "HalfLife07".
In HalfLife08 the bomb was well hidden but the bot somehow was still able to pick it up.
It's pretty hard dropping the bomb on "slope" floors so part of it will sink in the ground so I couldn't test that but i'm pretty sure they always pick it up when it's on those "slope" floors.
i must admit i didn't gave the bot alot of time but he was very close to the bombs in those events and even looked at it sometimes so i'm pretty sure i'm right. i'll test it again just to be 100% positive about this
oh, and you need to wait few seconds to picture to load after you click on it.