Can they take your keys incident to arrest? Yes. It sounds like you may have other issues regarding physical control. You should contact an attorney to discuss further. Good lluck.
If you are being detained in a police car that can be an arrest and the police can search you. More facts are needed about your situation. Hire a good criminal defense lawyer.
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The officer can take your keys if he suspects you may try to flee in your vehicle, or if you could use the keys as a weapon against the officer, property, or yourself.