Customer service 101

Nobody likes to wait in line at a grocery store, so as a cashier I feel under pressure to quickly handle the customers items and help the next customer, also as a cashier, I don't appreciate when people just leave their items on my check-stand and decide to leave without letting me know, so I guess you can say i've become a bit traumatized by the slightest experiences i've had by customers. I become so in tuned with my customers sometimes that I don't pay much attention the the customers after, which i've learned I probably should. The other day, I had a customer who put all of her groceries on my check-stand and left. She was gone for a few minutes before, I had another customer come through my line after, but since she didn't notify me that she'd be gone to get something I jumped to the conclusion after a few minutes that she might've left the store and left everything behind, so I put all of her stuff in her basket and helped the customers that had been waiting. As soon as I was done with a few, she comes back angry and frustrated with what i had done, but I didn't think it was my fault, after all it's not my job to read mind, it's my job to help my customers and get them out immediately. Of course I was reported to a manage, after she made my bagger put everything back on the check-stand and had me help her again. I gave her a smile and told her to have a great day.