When Holmes is in Gregson's office asking to come back to the NYPD, Gregson's phone changes position on the desk several times.
When Sherlock and Watson walk into the hotel room, a softbox light can be seen down the hall behind them.
Hotel room floors are not generally rated for heavy machinery. The one-ton magnet has a small footprint and could exert enough pressure to possibly crash through the floor of its hiding place in the bathroom one day.