Using awaymessage.tcl gives you the possibility to set an awaymessage that will be sent to the IRC server everytime you detach (close the connection to your sBNC).
If awaymessage.tcl is loaded, the awaymessage can easily be set using /sbnc set. It can also be locked for certain users if lock.tcl is loaded. Note that you can also set it in the web interface (if loaded AFTER iface.tcl of course).