The way I see it. I have opened the wallet containing the funds. Exported the private keys. Then probably by mistake I have used the sweep private keys (on the exact same wallet which had the funds). I don't think I've created a new wallet nor do I remember having any seed.
If you sweep the private key(s) into the same wallet, the fund should be sent back to the same wallet. Since the fund has been moved from your wallet, that doesn't seem to be the case.
I can't find any wallet file.
Did you check the default directory?
Where is the Electrum datadir located?When you export the private keys, there's a long list of keys. I saw that you can import the wallet using only private key. Does it make any difference if you use all the private keys or only one of them? I can't remember if doing that sweep I have used a specific key or all of them...
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There is no difference.
You can import/sweep any number of private keys.