What happens with my encrypted files on Synology when it is stolen?

At least with DSM 4.2, there is an option on the share dialog called "Mount automatically on startup." Make sure this options is unticked and if your NAS is disconnected (and stolen), it requires you to enter the password in the web interface to descrypt.

See http://www.synology.com/support/tutorials_show.php?q_id=455