I know some people, not me, that keep a spare key in their wallet. Now this would be
a key that has no big plastic on the end. Not sure if it is the master spare or if they
had one cut specifically. I should ask them. They are the type that would
take it out of the ignition and set it on the passenger seat while they
rummaged through something or picked something off the back seat, then
get out and lock the door. Just as they closed it, they realized what they
did.
I personally carry both sets of keys with me. It does make for some discomfort at times,
depending on what you are wearing.

So, I have considered the flat spare key idea.
This goes back to when I was doing field service work several years ago, and was using
my own car. I was 110 miles from home and locked myself out.
Not a good feeling.
