I stash my keys in the gas tank or under the hood just below my wipers most of the time. Most of the time I do this because I am within 30 min of my vehicle and riding light, no pack, and I hate having a bunch of stuff in my pockets. It is a convenience thing. If I have a pack on it is easy enough and plenty of room for them. Top the tire seems too obvious but consider most parking lots where mountain bikers are parking to ride aren’t frequented by the shady type. Most likely other bikers not looking to boost a car or steal out of it.