Cedric Charlier shows how to use server audits to track failed and successful logins (and logouts):
The core issue is that we have many users and logins on our databases and we have huge doubt their respective needs. The root cause is identified: sometimes, for a short period of time, we’re making exceptions to our own rules and let a few other friend projects access to our DEV database. On some other cases, we’re connecting our own solution in DEV environnement to the QA environnement of another solution. Why … planning, data quality issue, … we’ve valid reasons to do it … but these exceptions should be removed as soon as possible. And you know what? People forget. Nowadays, on our largest solution, we have 20 users but only 7 of them are expected and documented … other should be removed. But before executing this cleanup, we’d like to be sure that these users are not effectively used by other solutions. If it’s the case, we’ll need to update first the configuration of the corresponding solution.
Click through for a few scripts to show how to set this up as well as how to query the audit log.