To the best of your knowledge are the databases you used administered by someone who may have set up user group security? the reason I ask is that you may have had security set up before for another database, this security would then have been saved to your workgroup information file (this is a file that access creates the first time you open it) once user group security has been set up by a database administrator the details of your permissions e.t.c. are strored in this file, everytime you start access and try to open or create a new database access will look to this file for security details.
When using user-level security in an Access database, a database administrator or an object's owner can grant individual users or groups of users specific permissions to tables, queries, forms, reports, and macros. By using passwords and permissions (permissions: A set of attributes that specifies what kind of access a user has to data or objects in a database.), you can allow or restrict the access of individuals, or groups of individuals, to the objects in your database. Security accounts define the users and groups of users allowed access to the objects in your database. This information, known as a workgroup (workgroup: A group of users in a multiuser environment who share data and the same workgroup information file.), is stored in a workgroup information file (workgroup information file: A file that Access reads at startup that contains information about the users in a workgroup. This information includes users' account names, their passwords, and the groups of which they are members.)if you have not assigned a new workgroup that you control, then access will look to the default workgroup information file.
J Moore, July 2009