Sorry, I missed this. If you can't track the bees, perhaps the best thing you can do is get rid of what's attracting them. Can you eliminate the feed so they have no reason to come looking for it in your yard?Yea, I agree.
I've seen a few of them fly over a brick wall separating my backyard from an alley, but from there I'd have no idea.
I have checked my trees and I don't see/hear anything yet but I'll check again tomorrow.