Not an excuse,more a statement and question.
The big cage has no access through it, even a missing bar. In order to access it, you would have to cut a bar. It would also suggest by the "danger mine shaft" signs surrounding the shaft that going inside the cone was potentially a bad idea. It appeared very flat to me and I'm not sure what the floor is made of. There are bits of mesh covering the floor in places...not sure if this is waste, or what.
The other cage, covered over in a similar manner was also put in by operation minecap in the 80's and was also covered over by a tarpaulin. This was wrapped in a similar manner over an existing hole in the cone which covered an open and approx 120ft deep shaft with 5 levels off it.
Those cheeky chappies had been crapping down a shaft at Wheal Daniel, but the most simple reason tying these phenomena together are that "someone" was attempting to stop the flow of wildlife. Who this is and why they would want to do it would either appear, in light of a lack of evidence to be.
1. The developer of the site wishing to not consider habitats.
2. Someone doing a modern art installation.
Either way, a lack of evidence is a lack of evidence. Their report should note these features.....