It was the doorway. Walking through doorways empties your mind and causes forgetting. Really!
According to research conducted by Gabriel Radvansky and his associates, doorways create a disruption in our situational models, our internal pictures of our environments. In their experiments (both real and virtual) they asked participants to pick up objects and move them from one table to another, picking up a new object and moving it to another table. The distance travelled from table to table was consistent and the participants could not see tables and objects left behind from one table to another. Periodically they would be asked about the object they were carrying and the object they had just put down.
Whether in virtual environments or the physical reality, when people walked through a doorway they had more difficulty not only remembering the object they had just put down, but the object they were carrying as well. Radvansky also studied whether returning to the original room served as a memory jogger, re-instating memory. As it turns out... not really.
It seems that we construct situational models of our environment. They are not particularly specific because we are living in that environment currently. We don't need to hold a strong mental map of it because we can check in with our environment at any time to pick up more details. These situational models include not only information about the environment we are in, but also some vague information about ourselves within that environment (including insights into what we are carrying and doing). Doorways though are entrances into new environments which require us to construct a new situational model. This new model erases and over-writes the pre-existing model... causing us to 'forget'.
Changing the environment causes us to lose hold of the vague information that we have loosely been holding in our heads. Walking through the doorway causes us to replace this vague concept with another vague concept. Not early-onset Alzheimer's at all... just the doorway. So... the next time you are walking behind me and I pause just before walking through a doorway... know that I am not doing it to drive you crazy, just taking a mental note, reminding myself why I was going through that doorway in the first place!