Logins mapped to dropped database sql server
Try the following query:

SELECT * FROM syslogins WHERE dbname = 'ABC'

If there are logins still using that as their default database, this should help you find them.  However, if the users had a default database of master and the application performed a USE ABC command, then you'll have no way of knowing they were tied to the application.  It's only if the application was kind enough to create your SQL logins with a default database of 'ABC' will you be able to track things down.