If you haven’t locked the device down by the time they have instructed you to unlock it, doing then might be considered obstruction, but until they compel you to do it, you can do whatever you like with the phone.
As for destruction of evidence, it certainly wouldn’t be that, but even if it could be, it would kind of depend on them proving intent because you’re not under arrest yet or charged with anything. They would have to prove that you were aware of an impending investigation when you made the choice to dispose of evidence.
If you haven’t locked the device down by the time they have instructed you to unlock it, doing then might be considered obstruction, but until they compel you to do it, you can do whatever you like with the phone.
As for destruction of evidence, it certainly wouldn’t be that, but even if it could be, it would kind of depend on them proving intent because you’re not under arrest yet or charged with anything. They would have to prove that you were aware of an impending investigation when you made the choice to dispose of evidence.