The neighbors saw him in the house that day.
I think there is video of him sprinting out of that house he broke into. The neighbors alerted the neighborhood watch that Ahmaud was tresspassing inside that home. So it was seen by the other neighbors that Ahmaud broke in, and they alerted the neighborhood watch.
They had a gun because Ahmaud was breaking into that house in their neighborhood and criminals are often a threat.
I don't think they have proof yet that Ahmaud was stealing things in that house. But we do know he's been breaking into the house and that thousands of dollars worth of stolen items were missing according to the home owner.
The house being unoccupied doesn't mean the neighbors can't arm themselves.
Travis may have broke the law pointing the gun at Ahmaud, but the gun didn't go off untill Ahmaud grabbed the gun.
If Ahmaud was a normal good person, he would have stopped and admitted he shouldn't be breaking into the house. But that kid was not a good person. Him charging at Travis and grabbing the gun is why he got shot. He should have stopped once he was caught trespassing.