Is it normal for my base to change between sessions?

[Zep]

I am the only person with access. My characters only.

 

I have found floor levels changed -- or I am sleep walking to my base.

[WumpusRat]

Are you building above/below the actual base area?

[Zep]

Nothing outside the normal space -that I am aware of- it has only happened once so far.

[WumpusRat]

If it's only happened once, it may be that you were clicking on something else and the game registered the click elsewhere. I have that happen fairly often when I'm shift-moving objects up and down. Or it may simply be that you moved it at some point and forgot about it. I had that happen to me the other day as well, I moved some tables around, lowered the stuff on them, and came back later to find one object was hovering. I realized a few moments later that I couldn't remember having lowered that object to the table; I'd just forgotten to do so. 

[Zep]

It is, of course, very possible.