There are videos of him walking through the house with expensive tools in plain sight. Yet nothing was ever stolen.
It would have been a bit hard for him to walk away with anything substantial in broad daylight, especially since he was on foot. The owner of the property must have felt that his neighborhood was safe enough to leave everything in place, and the property unsecured. That was foolish of him, and set himself up to be a potential victim. Arbery had been there on other occasions, and could have been casing the place, with the intent of telling others about what was there. Nobody knows what the guy was up to. He was in a neighborhood he didn't live in, trespassing on property he had no business being inside. It shouldn't have surprised him that someone noticed his presence, and chose to confront him about it.