Absolutely, the "out of sight out of mind" syndrome is common for people who suffer from eating disorders. Absolutely. I see it all the time. And it is something that can be very much improved, if not completely eliminated.
The process of course takes time, because the sufferer needs to see over and over again that the therapist is consistent, does what she says she's going to, shows up at the times she says she's going to... that kind of thing- basically that she exists 24/7 and never fundamentally disappears for the client. How much time this takes depends on the degree to which the sufferer has the out of sight out of mind syndrome.
It's not just in a therapeutic relationship that this can be worked on. I talk to my clients about all their relationships, and try to find examples of where the connection continues to exist whether people are physically with each other or not.
Part of the difficulty is that if we didn't get to develop and solidify this "conserving of relationships" as youngsters we then go out into the world expecting all relationships to have this "poof" or "gone in a flash if I don't physically watch it every second" quality. Instead of entering relationships with the assumption/idea/belief that they do enduringly exist, we enter them rather tentatively and almost "waiting" for the other person to disappear.
Even in relationships that seem strong, we can live in a constant state of semi-panic, on guard and hyper-alert for any "indication" that the relationship has disappeared.- and feeling more and more like we have to spend every moment "monitoring" those relationships.
The key is collecting evidence. That's where therapists can be so helpful- we love to provide data (by which I mean healthy kinds of experience that is often different than what someone has encountered and experienced before) so people can revise how they see themselves and the world.
Don't despair, Wendy, your continued work on this will for sure pay off. And maybe it will help to now have a name (ok, a goofy name that I came up with, but what the heck...I think it works) for what you experience: the next time you leave friends and suddenly feel that horrible "rug being pulled out from underneath you" feeling, you can say, "oh yeah, there's that out of sight out of mind thing going on." This should help it not feel random, and can help you predict its occurrence.
And by the way, don't forget that your therapist is a real human being. It's true, there are guidelines, rules, ethical regulations even, for the therapeutic relationship, and those are all designed to protect and assist the client. In addition, therapists develop true, often deep, connections with their clients (I for one think I'm the luckiest therapist alive- my clients rock and I'm so fond of them). So if your therapist says she read something and was thinking about you, she means it- she's not required by law to say that. The boundaries of the therapeutic relationship don't at all preclude the development of real connection and relationship between clients and therapists. Not that you asked about that, but I couldn't resist :)