Achieving 100% completion in Metroid: Samus Returns means finding every single Energy Tank, Missile Expansion, Aeion Tank, and Power Bomb Expansion. The Scan Pulse is your most valuable tool for this. After acquiring all major movement upgrades (Space Jump, Screw Attack, Spider Ball, Grapple Beam), revisit every area. Use the Scan Pulse in every room, paying close attention to the map for any unrevealed blocks or item icons. Look for suspicious walls, floors, and ceilings that might hide destructible blocks. Power Bombs are often required to open up large hidden sections. The game's map will show you the percentage of items found in each area, guiding your search. check behind boss rooms and in areas that seemed inaccessible earlier.
