One thing I noticed is how close that location is to the freeway. While I'm not familiar with traffic patterns and highway noise in that particular area, I live in a building that backs right up to the freeway, the noise of which is audible even with all windows closed. Highway noise is a good cover for a lot of other noise IMO!
If someone brought a backhoe onto that property (to dig a hole big enough to bury a car in it), could it just appear to the neighbors that the lot owner was working to clear it/start a project?
I don't know who owns the property (has this been made public yet?), if that person lives on it, uses it for recreation/material storage, or if its vacant and sitting in disrepair, but I think its possible the neighbors might not have noticed the noise or activity, due to one factor or another. I pretty well keep to myself, and wouldn't know if someone was doing something nefarious on my own block. MOO.