Co-Worker Vs. Friend

Yesterday, when I was out and about, I ran into a former co-worker. She still works on the same floor I worked on for 5 years. You know what she asked me..."Are you still working there?" I was shocked to say the least. I looked at her with a puzzled look on my face and said "I LEFT 14 months ago!" She didn't believe me at first. She thought I was kidding. But I told her it was no joke. It is sad to say, but this isn't the first time this has happened. Back in the spring I ran in someone and she asked the same thing (I had already been gone for atleast 6 months). They both tried making up reasons why they didn't know I was gone: We worked different shifts, I worked usually only on Saturdays, Things get so busy at change of shift....blah, blah, blah. The truth is, they were only work acquaintances, not friends. I knew the difference and kept it that way. Sure, we'd talk sitting around the breakroom or the nurse's station but I would be careful at what I would say. They never knew the real me. I knew that once I left that job so would end the "friendship". I have had enough jobs to know how it works. Now don't get me wrong, I know there are some people that really are friends with co-workers and continue there friendship after one of them leaves their job. I have NEVER had that experience. I also never went out with anyone from work other than stopping for a coffee on the way home a few times. My free time was too busy. Also, hanging around with a majority of my co-workers would have meant I would have had to compromise in what I believe in. That wasn't happening. I guess what I am trying to say is the word "friend" is just too special to me and I don't like to give it to just anybody.