has a BTC thief ever actually been tracked down and caught before?
Hell yeah.
Trendon T. Shavers
Charlie Shrem
Ross Ulbricht
Neither Charlie nor Ross was caught for stealing. It was money laundering and drug dealing. As far as Trendon, I can't find any proof he was convicted of anything. But he may have.
SEC vs Trendon Shavers is still pending (AFAIK). The most recent web search that I could find gives a telephone number for those defrauded to call, so I presume it's still am active case.