[He's quiet for a few seconds, considering that and what they do know about how things work, including that people can leave and return and their memories seem dependent on location. But it isn't totally consistent; some people come back with no memory of being here before, after all.]

There are also... Not exactly 'normal', but known ways of losing and regaining memories for various reasons that are physiological, psychological, or both. For instance, repressed memories can often be regained through a specific trigger; if that's the cause, the Door might repress one's memories but a return to Hadriel could prompt the memories to return also.

[The subject is of course a complicated one, but worth considering.]