*chuckle* You are not alone. Every new FFE owner seems to go through this.
There are two things likely happening here:
1)
Your mind is playing tricks on you. Yes, you are simply forgetting to close the door and thinking you did. Seriously. This probably easily accounts for at least half of the occurrences you where you would "swear" the door came open "by itself". The longer you own and car and get used to closing the door every time, this issue will subside. (And it is even possible that these "mind tricks" account for almost all the occurrences.)
2)
You aren't closing the door properly, and so it isn't completely secured and is truly popping back open (with road vibration or whatever). If you are closing the door by "grabbing" the whole thing with your hand and twisting it back into place... stop it! That's wrong. The proper way to close the door is to use just your thumb (or finger if you like) to press directly on the little indentation on the door (which is there for this purpose). As you push at the indentation, the door will
automatically swing around and latch into place, and is how the mechanism is intended to work. It helps to be use moderately firm pressure when you do this and "carry it through" to the end... the door will depress a little into the opening, but return to being flush with the fender when you remove your thumb.
I swear... this "problem" will stop happening the longer you own the car. It never happens to me (and I thought it was happening "all the time" when I first got the car).
Good luck!
