Unfortunately, SNK cleaned up their act for Metal Slug 6 so there's nowhere near the amount of unseen areas as there was in 5. In fact, there's only one measly unsed screen, and they even had the courtesy to label it "unused" in plain English! What well-intentioned bastards!
Huge thanks to candy76041820 for the name translations.
The test stages no longer offer sweet stashes of all the vehicles and weapons in the game, but are actually used for testing specific screens. We've been conned, I swear to god.
FOR 5-05: ブラップ レイヤ
A copy of 5:5 that starts you right before the fight with the brainwashed ally. There are no objects, but it is otherwise identical.
FOR 4-01: ルーツマーズ
A copy of 4:1 that starts you at the fork in the road.
FOR 5-02: モグラスラッグ
Plops you before the drilling machines in 5:2.
FOR 5-06: ラスボス マェ
Puts you right at the final boss.
FOR 2-02: ローバローバノカベ
Mission 2, after you would have gotten the donkey slug's carriage.
A black screen full of garbage background tiles, the player flies around with the jetpack, but there are no enemies or obstacles. There is an invisible ground with a slight slope towards the end, but that's it. After a bit of scrolling you can leave the side of the screen and you'll transition to the start of Mission 2.