I Am More Observant Than I Let On

So the other day I was sitting in the living room watching my brothers play bloodborn and they probably didn't realize I was right behind them, because they started talking about how bitchy my mom is (which is true) and the listed off all kinds of bs she's done/ said. Well about three minutes in I joined the conversation saying, "Don't forget the time she threw a plate at the wall because no one made dinner that night.." Well my brothers both turned around and started yelling at me for butting into a conversation that I know nothing about... Seriously I was behind them the whole time. The aren't very observant, what if it was my mother behind them instead of me?