How you may know, Mate, that it was three different people? Because all that transactions to three different addresses? 
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I'm still pretty sure that someone had found some weakness or tricky way in how to recover private keys. But the way is not really easy because it takes several days to Mr. X to recover the private key, OR theway is really easy but the Mr. X has an ancient type PC and so it takes him several days to recover the private key.

Step "5" is not a sudden one, there is definetely something behind it! Otherwise if you have a way to recover the private key from four addresses with total amount of 3.7 BTC why wouldn't you do it???
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Also could be that there is a group of Mr. Xs who rent several VPSs with a huge RAM and distribute keyspaces between each other to search.
That would be quite an easy way for thouse who have money to invest and if the BS GS really the one which helps to find the private keys.
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In any way that Mr. X is genius compared to us, guys!  And 99.9% sure that Mr. X or Xs was/were here on this forum reading this topic.
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Dear Mr. X pls share with us at least the recently found private keys for our records.