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You don't need radar to track aircraft with ADS-B on: The plane is actively broadcasting its position.

There's ADS-B receivers the size of a USB stick - because some are USB sticks and available for 50 bucks on Amazon.

No, but if ADS-B is off then its not squawking unless its got like Mode-3 or Mode-S then maybe MLAT can be used.

Just to be specific, PAT25 seems to have been using Mode C for an earlier portion of its flight and Mode S for the later portion.