when is a door not a door?

Have singular experience with member of public who appears at my reception desk from corridor leading to gallery, points at door and asks Is This Entrance To Gallery.

I answer in the affirmative, as it is, while privately musing that actually the term exit would be more appropriate, and member of public leaves through door.

Member of public reappears moments later looking confused and says This Door Leads Outside. Am unsure of how to respond to this as A, it is true and B I'm not sure of source of confusion, as it is customary for entrances to be the link between outside and inside, and she was inside when she started.

Realise too late that Member of Public has walked through entire inside of museum looking for entrance to museum and can only wonder where she thought she already was and what she thought entire contents of museum, shop and free exhibition were.

Member of Public uses my hesitation do dart off down tunnel to special exhibition, which may well be what she has been searching for all along, but won't do her much good as she hasn't bought a ticket. As predicted she is turned back by colleague, but obviously considers contents of exhibition not worth frustration of talking to me again and sidles off back the way she came without buying ticket.

Do not care to speculate on her experience of Front of House helpfulness as aware to close an examination might reveal flaws in my own performance best left in dark, along with member of public.