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Google definitely indexes everything in the source code, and for some queries I have seen Google return the html mark-up in the snippet. But I've never seen UBBcodes in a snippet. UBB codes don't show up in the page source, unless of course that page is discussing the codes -- and then they're not part of the mark-up but part of the content.
Strange thing you've noticed - it does make me wonder how Google would ever get UBB coded source.