Am I paranoid? a quick trip to town this am

The store is not somehow magically obligated to take your worry away by becoming responsible for that child. If you were concerned, then you had the obligation to take action- either waiting there to be there to protect the child or bringing it to the attention of the authorities.
The fact that you called the person at customer service "the old lady" and were annoyed at her for not taking action to resolve your anxiety shows that you were reluctant to do something that you wanted other people even less involved to do- to fix the problem.
Just what did you expect the customer service "old lady" to do that you couldn't do yourself? Should she have made a public announcement over the PA there is an unattended child so please come and get her?
BTW I have become involved in situations similar to this personally.

A few years ago during the Christmas shopping season, my mother and I saw a child in the mall wondering around obviously scared and looking for someone. My mother and I stopped and spoke to the child and we decided that we would help. I sat down with the child on a bench in so no one would think we were taking her and my mother went looking for the mother in the closest store. Within 5 minutes we had the two back together. Thankfully, a happy ending. I don't blame mpillow for not wanting to approach the child alone. She could have been accused of attempted kidnapping or assault or anything and there would only be the child's word and mpillow's word. And I don't blame her for wanting to smack the parent(s) either.

My youngest son is 16 and I'm still concerned about him being by himself. (He doesn't know it though, not cool in his opinion.) You are definitely not paranoid. We "lost" my son (he was 3 at the time) at a softball game for about 10 minutes because I thought my husband had him and he thought I had him. They were the most sickening 10 minutes of my life. Sometimes kids get give you the slip even with the parents best intentions.

Since the child was on that store's property, they could have been held liable if something would have happened to her. Even though it's the parents responsibility. Stranger lawsuits have been filed and stranger ones have been won. The store should have been more concerned.

The difference in the stories later in this thread is that the person who was concerned did something- they sat with the child, talked to the child, took the child to the counter, etc. Simply going into a nearby store saying that a child is sitting on a bench alone outside the store is not effective action.
A lost child in the store is one thing- it's a pretty good bet the parent is in the store. But a child sitting outside? But an announcement saying, a child is sitting on a bench outside and will the parent please go get her? What if the parent is not in the store- you letting everyone know that the child is outside unattendend? What if the parent pointedly left the child there- you might not approve but?
If it would frighten the child to have a stranger talk to her, would it be better if the stranger was a store clerk?
No, the person who is concerned has to take the risk to make inquiry of the child.

I heard/saw an adult berating and beating a 2-3 yr old child in a WalMart parking lot, for "letting the new little puppy poop" on the (already beater-nasty) pickup seat. I yelled " Stop" and followed that adult- who was now dragging two little child- into the store and reported the situation to the manager. I guess I thought since it was on their property they needed to know... I was wrong. I was told they don't get involved but I could call the sheriff myself. UGH. They did give me the phone number, I called and gave a report, and a deputy came and took it from there. The sheriff knew by the license plate number I gave who the people were (small community) and knew where they lived.

I learned that if you see something... make the call yourself. I should have known.

When my daughter was 13, two high school boys approached her in the same Wal-Mart make-up aisle and began talking dirty to her, with one about to expose himself as I rounded the corner from just the next aisle over. I got there in time to see that something was going on with two strange boys standing too close to my daughter, and one with his hands down his pants. I spoke loudly as I approached, and the boys dispersed. I went directly up to the manager counter and spoke to him. Immediately I could tell he didn't want to get involved beyond asking me what the boys looked like...I complained that the boys were still in the store, and had even acting bold towards me, so he got the undercover shopper to follow them and then ask them to leave, and he escorted them out to their car. I followed up with a call to the local sheriff's who also followed up by asking me what the boys were wearing. They did go to the store and reviewed the tapes and said they thought they knew who they were ( one had a local high school tee shirt on). I know it was a basic he said she said until the tapes were viewed, and I was glad the sheriff's deputy took me seriously.

If nothing else, I learned to make the calls, my daughter learned not every friendly guy is safe, and that when you see something you do the right thing, and get involved. It is a bit like "Deliverance' around here so you need to use some brains too- but I'll not sit back and watch a child be hurt ( and I do not mean basic paddling type discipline here) because I was afraid to speak up.

I turned a shoplifting skanky girl in just Saturday afternoon, when I saw her reach into her purse so slyly with something in her hand., coming back out of the purse with her cellphone. She knew I saw her.

-Scrt Crk

Children are generally more perceptive than we think about danger. The human race would have long since died out if God had not given children at least that much sense.

I'm a hoverer, I guess, to use Stan's word. With small children anyway. I can't stand to see a mother walking along in a parking lot without holding the hand of the small child beside her. I'm occasionally late because I saw someone like the girl the OP described and I sat down to guard until they were safely away. I don't worry too much about teenagers usually, particularly the ones who are up to shennanigans. So long as their shennanigans keep them far away from me. And teenage boys tend to respond differently to men most of the times. About 99% of the time they simply scatter in front of you and never cause any problems.