I think he means they will have your monthly bank account statements, which is true. We all know that if hmrc have reason to suspect you then they will access all of your financial information. They don't however monitor your bank account month to month, that is utter bollox.
When I went self employed in 2010 I had nothing. I borrowed money to start up the business and I was claiming working tax credits whilst I built. Of course I informed the tax credits office (hmrc) of the loan, which they allowed, as I only had the money in my pocket for a couple of days before it all got ploughed into the business.
A couple of months later I receive a nice letter through the door telling me I was being investigated for tax credit fraud
I had to attend an interview under caution, where they had all my financial info from the last couple of years in front of them, with certain "suspicious" entries highlighted.
Of course little old me was completely innocent and had an explanation for everything, to which the lady eventually agreed I had done no wrong doing.
So anyway at the end of the interview the lady turned out to be lovely and we got chatting.
I asked her what caused my investigation, of course she couldn't tell me directly, but did tell me that all investigations begin with any sign of suspicious activity, including tax investigations. That can be through someone snitching on you, discrepancies in your yearly tax returns i.e Wildly fluctuating turnover, huge amounts of expense for the type of business run, even late payments can cause an investigation, amongst other things.
I also asked about how they accessed my bank account, she told me that once they begin an investigation they are allowed to access this information. I asked if they always monitor everyone's bank accounts, which she laughed at and said no, only once an investigation is opened can they legally access your financial information.