This statement reminds me of the concern over the murder weapon during the OJ Simpson trial. A lot of people felt that it was absolutely essential that they have a murder weapon. They acted as if not finding the murder weapon absolved OJ of any culpability.
My point at the time was that the murder weapon did in fact exist, because the people were murdered with it. The fact that they were murdered with a knife (Believe it or not a Swiss Army Knife) demonstrates beyond question that the weapon existed. Not being able to find it did not make it nonexistent, it made it non available. It still existed.
Now apply that logic to the McMichaels. If they were not aware of his trespass, then how the f*** did they know to be chasing him? Clearly they were aware of it. Not being aware of it would have caused them to do nothing. The mere fact that they did something proves they had knowledge of the trespass.