Regardless whether someone knows or not if there is a passphrase, they could suspect and try to guess it anyway. The point of having a long passphrase is that even though someone tries to access your wallet through brute force, they would not be able to.
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If the 12 or 24 main words constitute a fake wallet, how much should be left in it? I suppose it depends on the standard of living in different countries? And how much would you leave in your fake wallet?
I would not want to put so much obviously but putting too little can fool nobody either. So finding a good balance between what I can afford to lose that can be used as a deception to hackers is the key.