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The Unknown Signal was obtainable in Season 13: "Become a Supervillain" by reaching level 6.
Unknown Signal's current MU count is at ~173, which is a bit higher compared to most other rare items (such as Champion Trumpets or Sled Void), however its still incredibly difficult to spot in trading, way harder than majority of the uncommon items.
Supply is widely distributed — no single holder owns enough to register as a hoarder.