Keep the comments coming.
I'm real interested.

 

As a teen, I carried a spare key for my VW Beetle in my wallet. My dad, with Alzheimer's, taped a spare key to his windshield wiper arm with electrician's tape. It was rather unnoticable. Consequently, I believe he forgot the key was there.

The $2 spare in my wallet saved me once, but it would be hard to justify a $500 spare today. A few years ago I needed a locksmith to get in my car. It cost $60 and my insurance company reimbursed me.

 

I have on occasion carried spare keys in a magnetic key box. In one case when I checked to see if it was still there it was not. Apparently over time it had come loose and fell off. Other than that I don't see any reason not to hide a spare key on your car although I would not trust the magnet to hold it and I would attach a safety wire as well. If you are worried about car thieves most have far easier ways to get into your car and I doubt that they would spend the time looking for a hidden key.

 

When I bought my 4Runner, they included a plastic credit card key and they cut it for me. I kept it in my wallet. Now that my wife drives it, I gave it to her. Don't ask me which of her many purses it's in these days. Is there something like that for the Fit you can find?

Fortunately I never had to use it in an emergency, but I did test it out and it works the door locks and obviously cannot start the car because of no transponder.