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If it doesn't exist, it's out of mempool already.
Ooh! TIL 
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This is weird. I suggest to backup your wallet.dat, and start Bitcoin Core with command line options -zapwallettxes and -rescan. This removes all unconfirms transactions and reloads all confirmed transactions that belong to the addresses in your wallet.
I'll do this and update you with results.
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Maybe. But what thief steals only a part of your coins?
Have you ever exported your private key? Can the transaction have been made from another wallet with the same address?
Have you ever exported your private key? Can the transaction have been made from another wallet with the same address?
Yeah if they had access they would of had a nice haul from me, and my wallet would have displayed empty when it resynced. I've never exported the private key from this wallet to another, and we cannot rule out the possibility that this tx may have been made from another wallet with this address.
Thanks for your reply, and I'll see what i can do with this information!