Brock Lesnar's return to WWE on Monday night was kept a major secret backstage. There were only a very select few people who knew he'd be appearing on the show. We're told Lesnar did not arrive to the arena until 9:30PM ET.
The details on his return were kept quiet from most backstage, so the ending of the show (where he F5'd Punk) came off as a genuine surprise to most of the talents backstage. John Cena, CM Punk, Vince McMahon, Triple H and Stephanie McMahon all knew about it beforehand but other than them, most everyone else was unaware of the planned angle.
As noted earlier here on the website, WWE is planning to feature Brock Lesnar vs. CM Punk at the Summerslam PPV in August in Los Angeles and not at next month's Money in the Bank PPV. The angle on RAW this past Monday night was a way to start getting things going with the feud between the two.