I think your concerns are valid. I'd be more than a little freaked out if I handed someone my credit card and suddenly it went missing, if only for a few minutes.

I agree talking to a manager at the time of the incident would've been the best option, mostly because he/she can do something right there (like talk to the employees about the proper way to handle credit cards). On that note, I also want to point something else out. In your letter, you only reference your local McDonalds. Corporate doesn't know which McDonalds is your local one. It might be best to rewrite the letter referencing which McDonalds you are speaking of.

But on the whole, I agree the employee didn't handle the situation correctly. I'm glad they were able to find your card, though. It would've been much worse if they didn't.