This so resonates with me. When I take a trip to visit my sister out of state, I'm supposed to call my husband when I get there, but by the time I get there I have 'put him out of my mind'. The longer I'm at my sister's the less attached I feel to my husband.
This also happened to me when I was hospitalized (which lasted a month or more). I didn't miss my husband or friends or family. I just exist wherever I am.
As far as the friends that are in my life now, it doesn't usually dawn on me to call them. Out of sight, out of mind. I have been using a 'tool' though that helps me keep connected to people a bit more (Facebook).
I could go weeks or longer without talking to anyone but my husband (and he and I don't talk much). When I get around a person I think about how 'close' I feel to them and wonder how I could forget them so easily. But I do.
So, I totally relate to what you are saying. Usually it doesn't bother me because I'm not really aware of it until the person is around me again. Then I might think about it, but I let it go just as easily.
I think I'm like this because I never got attached to my mom. She always pawned me off on my grandmother (her mom) so I never figured out how to develop relationships. I've only had one friendship in my life that was long distance (a current friend who lives in another state). Mostly she calls me but we do talk a few times a week. If something happened to her I don't know if I would miss her or not.
That's another issue I have - when people die, if they are not in my every day life, it's hard for me to feel any kind of loss. Logically I know there is a loss but I just don't feel much about it. I had 2 people in my life die in November and I've pretty much forgotten about them already. I know it's a coping mechanism, but it's a crappy one.
Anyway, I just wanted to let you know that I understand what you are talking about.