First and formost, when you leave the key in the car, the fob "talks" with the ignition unit and your
key's battery will drain faster than in normal use. (The owner's manual states in several location to "not leave the key in the car".)
Also, in some instances, because of the way one leaves the vehicle (what is done and in what order, i.e. open the door BEFORE turning the ignition off), the security system will lock within a minute (I have to do more experiments on how this actually happens).
Another issue is where the key is stored: in one's pocket, its button(s) may be pressed unknowingly, causing the transmitter to
lock the car even through walls.
It is a good idea to
always take the key out of the car (if just to save on its battery life).
Another suggestion is to switch key now and then, to keep
both fobs in regular contact with the ignition (to allow them to refresh their common code memory), and to even out battery usage.
Note: a kid playing with the key when it's out of range of the ignition may also result in the fob to lose track of its codes (is it 256 hits, and the codes are lost?). Re-programming the ignition system is a costly proposition, requiring a Dealership's intervention, with a connection to Smartville.