Sirius XM Holdings (NASDAQ: SIRI) hit 11-year highs earlier this month -- fortified after posting another blowout quarter in late July -- but it doesn't mean the naysayers have gone away. There were a whopping 266.5 million shares of the satellite radio provider sold short at the end of July, its highest short interest on the investment since mid-March.Short interest on Sirius XM has risen by 20% over the past year, but that's just one data point.