MSDN:
For each person who opens a shared database, the Jet database engine writes an entry in the .ldb file of the database. The size of each .ldb entry is 64 bytes. The first 32 bytes contain the computer name (such as JohnDoe). The second 32 bytes contain the security name (such as Admin). The maximum number of concurrent users that the Jet database engine supports is 255; therefore, the .ldb file size is never larger than 16 kilobytes.
When a user closes a shared database, the user's entry is not removed from the .ldb file. However, the user's entry may be overwritten when another user opens the database. This means that you cannot use the .ldb file alone to determine who is currently using the database.
Alas...