Access uses a workgroup file (mdw) for every session - doesn't matter
whether your mdb is secure or not.

It ships with a mdw named system.mdw and uses that, silently logging you in
as 'Admin' user.

When you secure a mdb, you create a new workgroup file.  Access made this
secure mdw your default.  Use the workgroup administrator and rejoin
system.mdw as your default.  That will require no login when you start
Access.

For the one secure mdb you have, launch it using a desktop shortcut which
uses the /wrkgrp switch to over-ride your default mdw *for just that
session*.  The target would look like
"path to msaccess.exe" "path to mdb" /wrkgrp "path to secure mdw"