Hiding your key
Answers so far are good and well-reasoned.
I hide a key on my bike for different reasons. I'm not worried about thugs stealing my bike--such worries are unproductive. It will happen or it won't. And it wil kill me. So......
I hide a spare key because I go on road trips, usually 10 day deals. I know I could lose a key. This is well-proven. Therefore I do this:
I clean my fairing well with the usual cleaning agents. I'm talking about the right-hand outside cowling. Just aft of the steering column/gas tank opening
Under the lip, between the gas tank and the fairing you can reach down and up. Tape your key there with duct tape.
Mine's been there for three years. I check it each year. No rust.
I used to put it in a Ziplock bag before taping....that was on my car. The key rusted off to rust hell. It was a BMW car and BMW key.
Never will be without a key this way, even if you stagger out of the bar with no wallet and a bleeding nose. If they've stolen your bike, well, a gross of spare keys will get you nowhere.
What's the key?
Ensure that the spare key is where it should be before each ride.
What could go wrong?