You can check the default trace if it happened recently enough. This query (stolen from RedGate) will work. 

SELECT TE.name AS [EventName] ,
   v.subclass_name ,
   T.DatabaseName ,
   t.DatabaseID ,
   t.NTDomainName ,
   t.ApplicationName ,
   t.LoginName ,
   t.SPID ,
   t.StartTime ,
   t.RoleName ,
   t.TargetUserName ,
   t.TargetLoginName ,
   t.SessionLoginName
FROM  sys.fn_trace_gettable(CONVERT(VARCHAR(150), ( SELECT TOP 1
                     f.[value]
                  FROM  sys.fn_trace_getinfo(NULL) f
                  WHERE f.property = 2
                  )), DEFAULT) T
   JOIN sys.trace_events TE ON T.EventClass = TE.trace_event_id
   JOIN sys.trace_subclass_values v ON v.trace_event_id = TE.trace_event_id
               AND v.subclass_value = t.EventSubClass
WHERE te.name IN ( 'Audit Addlogin Event', 'Audit Add DB User Event',
       'Audit Add Member to DB Role Event' )
   AND v.subclass_name IN ( 'add', 'Grant database access' )


If it's not there you can try looking at previous .trc files as this only looks at the current one. If you check the results of the "select top 1" subquery you'll get a result like: C:\MSSQL\MSSQL11.MSSQLSERVER\MSSQL\Log\log_124.trc. Use that result to replace the subquery with the previous file (log_123.trc in this case) and work your way back as long as you've got files.


SELECT TE.name AS [EventName] ,
   v.subclass_name ,
   T.DatabaseName ,
   t.DatabaseID ,
   t.NTDomainName ,
   t.ApplicationName ,
   t.LoginName ,
   t.SPID ,
   t.StartTime ,
   t.RoleName ,
   t.TargetUserName ,
   t.TargetLoginName ,
   t.SessionLoginName
FROM  sys.fn_trace_gettable('C:\MSSQL\MSSQL11.MSSQLSERVER\MSSQL\Log\log_123.trc', DEFAULT) T
   JOIN sys.trace_events TE ON T.EventClass = TE.trace_event_id
   JOIN sys.trace_subclass_values v ON v.trace_event_id = TE.trace_event_id
               AND v.subclass_value = t.EventSubClass
WHERE te.name IN ( 'Audit Addlogin Event', 'Audit Add DB User Event',
       'Audit Add Member to DB Role Event' )
   AND v.subclass_name IN ( 'add', 'Grant database access' )