My guess is that if you have a bike rack on your car, people familiar with cycling on a trail would know that you don't have a lot of room on a bicycle to take your valuables with you. Therefore, even if they're covered up, if you're parked at a trailhead with a bike rack on a vehicle.. you probably just took your keys, phone, and wallet with you and left anything else of value behind..
This is partly why I try to bike TO the trail to start my rides - then I don't have to worry about losing much!
“Hope is the thing with feathers, that perches in the soul, and sings the tune without words, and never stops at all.”
- Emily Dickinson