In all likelyhood Buffett probably never owned Iron Mountain to begin with. The SEC filings indicate Berkshire Hathaway owned shares but not who made the decision. It appears, as with Comcast, that IRM was owned by the Geico subsidiary. Which is Lou Simpson's investment. He is a good investor too, but not in Buffett's league.
True: approx 2 million of a 7 million share position: "The firm decreased its holdings in Iron Mountain Inc. to 5 million shares from 6.9 million shares in the third quarter." Per Marketwatch article this morning
Yesterday, Morningstar highlighted the stock as a "Company We Love".