Is there a name for knowing something is gone but not remembering what it was?
Sometimes I'll notice that a building that I used to drive by all the time is gone (leveled to the ground). I know it's gone, but I can't remember what it looked like or what was in it. I just know it's gone.
I have that HARD. As I'm heading out the door, I have a mental list (grab keys and that paper for work, stop for gas, message person about thing) and if my wife asks an unrelated question on my way, I essentially rip up the list, answer her question, go out the door, head to the car, come back in for my keys, get to work, notice the empty gas tank, and remember that paper I left at home.
Yep, then you walk into another room (new context) and poof, it's gone, but sometimes you can go back into the other room and you start to remember. I'm no expert but I believe that's all related to context dependent memory.
Another trick can be to chew a certain flavor of gum when studying for a test, then chew that same flavor of gum while taking the text: It will help you remember your studies.