Good evening pickel
No, there shouldn't be a way that sheets would be somehow unlocked when you send them out ...
however
... Excel's security on this front is scandalously feeble and can be undone by someone who doesn't know the password in less than 30 seconds - perhaps one of your customers is just being a bit cheeky / mischievous in letting you know that they have removed your protection.
HTH
DominicB
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