Hi Fabio,
Ok, I think I understand. It must be that NWBC is detecting that it is being opened in a frame (rather than in the main browser window) and removing the logoff in those scenarios. I would probably start by looking at the JavaScript (using the browser debugging tools) and trying to see if there is anything that is removing the logoff. The hard bit will be that the JavaScripts for NWBC aren't actually stored as files on the server that are easily modified... see my reply here:
http://scn.sap.com/message/16025468#16025468
In SE80 the main NWBC classes are /UI2/CL_NWBC* the you should find the RUNTIME and HTML classes, sorry I am not in front of a system, so I can't give you the exact class names. But this should help you find them
Hth,
Simon