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It does not appear you have to volunteer the tape to the police, but if
they ask for it and you don't indicate you have it, you could be charged
with obstruction of justice or withholding evidence.
Even though it directly speaks to murder, this "Slate" article by Laura
Hodes addresses your question as well.
"Can a Witness Who Fails To Out a Murder Defendant Be Charged With a Crime?"
"To be prosecuted for obstruction of justice or withholding evidence, someone
with knowledge of a crime must lie to a police officer, either by fabricating
or withholding information.
In Illinois and most other states, it's not a crime to fail to report what you
know about a crime; the law doesn't compel you to come forward and report a
murder. Similarly, state statutes also require an affirmative act to label
someone an accessory after the fact. You can't be called an accessory just for
Please note the "most other states" in the excerpt above, as this might indeed
be different in your specific state.
If you need any clarification, please feel free to ask it.
Google search on: "withholding evidence" crime law
Looking Forward, denco-ga - Google Answers Researcher