Walkthrough: After finishing Area 3, you can head back to Area 2 to not only find collectibles, but also an Aeion Tank and Missile Expansion.
Missile Expansion: Go to the lava room where the Teleportation Statue is. Use your newly acquired Burst Bea, on the Egg you find, and then Bomb the weaker point to find the hidden Expansion.
Aeion Tank: Head back to the long tunnel on the east side of Area 2. Find the weak point in the wall at the bottom of the tunnel, and Bomb it to access a small room. Use your Spider Ball to reach the top of the room, and then use your Grapple Beam to pull Samus to the other side. The Aeion Tank is just down past here.