It is the address that matches the "sample" private key listed on the Bitcoin Wiki: 5Kb8kLf9zgWQnogidDA76MzPL6TsZZY36hWXMssSzNydYXYB9KF
People occasionally send a few satoshi's to this address and they are generally moved within minutes (if not seconds).
Given the speed with which your coins moved (approx 2 seconds)... either your wallet software is "bad"™ and is configured to automatically send coins out... or your seed/private keys have been compromised somehow and the thief has an automated script monitoring the addresses for deposits.
If so, have you checked and verified the digital signature of the file? This is the only way to guarantee authenticity of the wallet.