A popular method used by thieves is to have the least threatening one distract you while the other one lifts your wallet from your purse. Often, the victim does not even know it until she goes to pay at check out. Purses are targets. I will put my hand on my purse anytime I have to turn my head away to look for items on the shelf and I keep it zipped up.
On a related note; the theft of cell phones from purses and even right out of your hands is becoming common also.