"SBTC" means nothing, and was simply pulled out of thin air by someone who wanted to give an air of mystery to a mythical intruder belonging to a "small foreign faction."
expat79, this is my suspicion as well. I believe it was just random letters chosen to form an acronym and further confuse/confound police to get them chasing more red herrings. And let's not forget, PR was especially fond of making up private little acronyms that she used in her correspondence. For example, a Christmas note she wrote to a friend was signed "P.P.R.B.S.J." -- which stood for Patsy Paugh Ramsey, Bachelor of Science in Journalism.