Mike and everyone else knew they were there. Remember Mike offered to feed Johnny's men.
Did Mike see ALL of Johnny's men? When Mike sees the dead assassins lying in the gutter, he is obviously NOT seeing the same men that were in his office that afternoon. Otherwise, he would know Johnny is behind the plot.
I don't know exactly how many men Johnny Ola bought with him, I suspect just two and they were both with Johnny came into the house, and Mike probably didn't pay too much attention to them.
What leads you to believe that Mike didn't recognize the men in the ditch? I don't remember it being shown that he even looked at the men, and even if he did, would he have told anyone that they were Ola's men.
The fact that he knew Roth was the one that tried to kill him, tells me that he knew Ola was in on the plot.
