I assume they have interviewed and grilled all the employees of the surrounding businesses. The place all of this happened looks like a good hang-out spot for people that want to get hidden and smoke a little dope before work. This occurred at 7:50 A.M. ----10 minutes before many businesses open.
There are numerous businesses sprinkled right there in the vicinity of the old concrete plant. To use as an example, let's say we have a group of "friends" that go to this place before work, during break, and at lunchtime to meet up and smoke a little weed.
They may, or may not all work together. By the time Gliniewicz's back-up officers got to the scene, all three of them would have been at work for the day right along with everyone else. They may have never let on that anything ever happened.
Map here:
https://www.google.com/maps/d/edit?hl=en&hl=en&authuser=0&authuser=0&mid=zAAmDUXJe_aE.kdNsSY53CGeU