Well. the question is why your device is locked to a user, who you don't know?
How did you get it? Can't you contact the person who is owner of the account and who obviously owned the device before you?
For cracking the lock I can't help you. I never did this.
Funny thing, long story. I've the phone (was new not a used one), I have SIM (with the number which is locking the phone) but ID of the Xiaomi account doesn't match mine. I only have one e-mail and Xiaomi account and is not that. I tried to recover an account with my phone number to see what happens and I recovery, as I expected, my account no that blocking my phone. The funny thing is I dont have a recovery phone added to my account and when I try to add it, it doens't allow me cause is already associated with other account (the one blocking my phone I assume) but when I try to recover and account it recovers mine! the one I don't have a recovery phone! :S
That why I said that is not my account and nobody (at least I know) played with my device. I used the Xaiomi recovery forms (they are only available for China, Indonesia, twaiwan not Europe because they ask me for a picture of me holding my personal ID in the form so I pass... and I only can select that my phone was bought on one of those countries and then try emailing to
service.global@xiaomi.com and the one in my country too. All generic answers of "try to recovery your account bla bla bla" I submitted pictures of the phone locked, original boxes, IMEI, unlock code on screen bla bla bla... nothing.
I one of those forms, I try to recover the pass of my account by using the phone too but those forms showed me more info (the ID of the unknown account less verbose and I see that there isn't any e-mail associated to it so WTF?, maybe a deleted account or something rare? I do not created any other. If I try to put my email then I see my ID and my e-mail associated, but no phone... as I expected cause that phone is associated to the unknown account. I tried to sent the apply anyway with the info it ask me (IMEI, phone model...) but they sent me an SMS notifying me that They couldn't verify my account (if I look the SMS the unknown account ID digits appear on top, less verbose but still hidden part of it). Something strange happens with my account... I don't know if I explained well...
Of couse in my account in "find devices" I don't have any phone associated with "find device" enabled... is associated in the unknown one probably.
Yeah, quite crazy, so that's why I doing the custom ROM thing. I don't use Xiaomi cloud anyway and this phone is around 2 years old... but is incredible Xiaomi doesn't allow me to unlock it because I have it all but the ID of that unknown account...
EDIT: Other possibility I think of from reading again your post it is that this phone was used in that shop I bought it (a physical one) and locked there cause my sim was bought there I think I cannot remember, was 2 years ago and I don't remember I did my carrier contract there. I wasn't a contract phone though was bought in one payment.