Does missing logout event in SQL Server trace mean connection leak?
Great question! To find out if the connection is still open right now, look in sys.sysprocesses:

SELECT * FROM sys.sysprocesses;


Generally, as long as they're not doing anything, open connections aren't a problem for SQL Server - it can handle tens of thousands. Connection pooling - keeping a connection open and ready for reuse - is fine, and doesn't consume that many resources on a SQL Server.

I'm generally more worried about sleeping sessions - someone who started a request minutes ago and didn't finish - and you can find those with Adam Machanic's free sp_WhoIsActive:

sp_WhoIsActive @ShowSleepingSpids = 1