As noted in RHa's comment, it has nothing to do with the properties of sein but how außer is used. If you use außer wenn then the verb goes at the end since wenn is a subordinating conjunction. If you leave out the wenn then außer seems to act like a coordinating conjunction and the verb goes in second position (after Sie in this case). You seem to be using außer to mean "unless", but this use seems poorly documented at best. Bruce Duncan's site says specifically 'German has no word for "unless."' He recommends using a special subjunctive phrase es sei denn. But, at least in this case, you could use wenn with a negation, keeping in mind that wenn is still a subordinating conjunction and pushes the verb to the end: Hallo, treffen wir uns am Donnerstag (wenn Sie nicht im Urlaub sind)? Despite what Bruce Duncan says though, I could find quite a few examples where außer (as a coordinating conjunction) is used to mean "unless". Note that "unless" is a subordinating conjunction in English since a clause using it can be placed in front of the main clause. But by this criterion außer is a subordinating conjunction in German as well, going by the example (from NCIS subtitles) Außer Sie sagen uns, wo er ist, haben Sie nichts anzubieten. So, to tell the truth, I'm a bit confused on this issue myself. Perhaps this is an example of Denglish; subtitles aren't always the best examples of correct grammar, though they are often a better indicator of the way people actually talk. Anyway, I'm not sure if this counts as an answer so perhaps you should regard it as an extended comment.
