Doors Unlock

[LORI D]

I've had my 2007 ES350 for 6 weeks and during that time I have had this scenario happen a dozen times. Can't figure it out.

I get out of my car and use the button on the door handle to lock all the doors. I walk away and after about 5 feet, the beep goes off and the doors have unlocked themselves. My keyfob is in my purse like always and I don't see how the button on it can get bumped. Any ideas???