First time I heard about this. Where has this been reported?
Just some tinfoil hat stuff that I found interesting. Now, with all the other recent disclosures of FBI shenanigans, it seems entirely probable. I first read of it in the Wash Examiner when they reported on a piece DiGenova had done on it. Then, right after the Mueller report was released I recall a congressman (maybe Gowdy? can’t recall for sure) say that he was interested in getting Mueller under oath to ask about the phone. Since then, not a peep....here’s the link: https://www.washingtonexaminer.com/news/joe-digenova-shares-conspiracy-theory-about-fbi-using-mueller-phone-to-secretly-record-trump
Here’s another link to the bit of info McCabe dropped in his book. I am sure this was not some throw-away line and bet it meant something to somebody. https://www.breitbart.com/politics/2019/02/25/book-robert-mueller-left-phone-inside-after-trump-oval-office-meeting/