Avalon (training mission): Secret Nali shooting range
Once it is time to mantle onto the higher block in training, jump over the fence to your right. You should see an open crate with a weird head inside. Pick it up to get a "You unlocked the secret shooting range" message. When its time to test the weapons, specifically the grenade launcher, move to the left (as if there was another weapon to try out). Another shooting range will be unlocked. It is Nali. The cardboard figures even talk when you kill them.