I prefer to leave my keys at the car. Just that dreaded thought of returning to my car after a 20+ mile hike only to find that somehow someway i lost my keys on the trail would be a horrific experience that i plan to avoid. Especially if you are several hundred miles from home in a very remote area, which is a common practice for most of us hikers

But i also understand that areas like trail heads and long term lots are like a magnet for criminal activities. I have never had any bad experience but i am sure there are plenty out there that have. I also drive an old beater probably the last car someone would want to try and steal lol.

The new key-less start vehicles which are becoming the new standard however i think could pose a threat to both sides of the fence, since water (possibly even dew), sub zero temps or even dropping your pack off your shoulder on a hard surface could cause it to malfunction. I guess only time will tell but for the foreseeable future i think i will continue to hide my keys at the car.
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Leave nothing but footprints
Take nothing but pictures
Kill nothing but time