A quick side note - the <b> tag is not formally deprecated. The <u> tag, for example, is deprecated, but <b> is not. If a character string is marked up to be given semantic strength, then <strong> is recommended. But <b> is still used when the mark-up indicates a typesetting instruction with no semantic intent. Of course, CSS is the recommended method.
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.