Just checking, but you do realize that the key memory is only activated when you lock and unlock the car, right? I didn't see anything in your post indicating that, including your comment about the button.
Basically, when you lock the car with the lock button on your key, it stores all the current memorizable settings. When you unlock the car with the unlock button on that same key, it restores those settings. If someone else locks and unlocks, it does the same. There's an option whether the resetting takes effect when unlocking the car or when opening the door, but you have to lock it in between for it to store and reset.
As far as the memory buttons go, they are independent of the key memory settings. The key settings store whatever was set when the car was locked; the memory buttons store whatever the settings are when you go through the storage procedure (press the M button, then press the desired memory position, IIRC). So, even if you lend someone your key, it's easy to get back to your preferred settings as long as you stored a memory position.