The Metroid Detector (also referred to as the Metroid meter in some print) is a technology exclusive to Metroid II: Return of Samus. It appears as a small Metroid in the bottom right corner of the screen, and is used to detect the number of Metroids remaining on planet SR388. Whenever Samus Aran kills one, be it an Alpha, Gamma, Zeta or Omega, the meter will scramble for a few seconds before it decreases the number by one. If the game is paused, the tiny Metroid will be replaced with an "L", indicating how many Metroids are left in the area.
By default, the number is thirty-nine Metroids, however, when Samus is approaching the Queen Metroid, crossing the first scene with the Metroid Egg in Phase 9, the meter raises up to nine, indicating the eight mature Metroids. Although Samus may return after escaping the battle with the Queen, the eight Metroids will not respawn.
Strangely, when the baby is born, the meter does not raise up to one. This implies that the Metroid Detector does not take into account Infant Metroids (for unknown reasons) until these grow to the Larval Metroids more commonly seen in other games. This could also be due to an error in the code, the hatchling's inhostility or because it was never scripted at all. Discounting the hatchling, the meter totals forty-seven Metroids throughout the game.
- The number of Metroids detected on the planet.
- This detector can also show other information about Metroids.