My private board was accessed by my office manager.

[Brent Ackors]

I am the only member on a private board but somehow my office manager was able to get into it and read my personal stuff.

How do I make it truly PRIVATE.

[Hannah Humbert - Simpla Workflows]

Do you have a premium workspace? If so, any workspace admin can see all boards within that workspace, regardless of if it's private or not. (see documentation here).

If you want to make the board truly private, you might want to look at creating a separate private workspace and then move that board into the separate workspace. Just note that if you're moving a board from a premium workspace to a free workspace, you may lose some features seeing as the billing is at the workspace level.
 

[Nic Brough -Adaptavist-]

The only way a board can be shared with people is for the owner to make it available to them.

Check the permissions settings on your board.  If your office manager could see it, then you must have set it up for them to see it.