openssl s_client does not indicate that SQL Server is using TLS, however the connection is secure. How is that possible?
Have the DBA run the following query to see if native SQL Server client-server connection encryption is being used.

SELECT @@SERVERNAME AS 'sql_server', a.host_name AS client_host, 
    a.login_name, client_interface_name, b.encrypt_option
FROM sys.dm_exec_sessions a 
JOIN sys.dm_exec_connections b ON a.session_id=b.session_id
WHERE login_name = '&lt;login_name&gt;'


If the encrypt_option column is false, then it's not using the native encryption.  At this point, any encryption that is wrapped around the connection would have to be at the networking layers, either Windows IPSec (or something similar), or encryption implemented by the network devices.  So you'll have to ask the DBA exactly how the connection is encrypted in that case.