Urban exploration is fascinating to many people and there are many social media venues, blogs, websites, etc where people post their adventures, finds, etc. The father of a friend of my son's is into this and will access old abandoned buildings, etc. I've always wanted to go but I'm afraid of getting caught/arrested, but I do love to see his photos/videos. Unfortunately, these places are often hazardous and serious injuries and even deaths have occurred.
I'm not saying that Caleb was into this as there isn't (to my knowledge) any concrete evidence of this. But one thing REALLY stood out to me with his final post of the bridge in the fog - Caleb appears to have a "photographic eye". What I mean by this is that he sees things as if looking through a camera lens and instinctively can visualize a "good picture". I have this and will stop and photograph something that jumps out at me even though most other people may not even notice it. This can be an amazing gift, but it can also be dangerous if you are impulsive and get so focused on getting the picture that you may not notice potential hazards. I think we can all recall stories of someone stepping over a guardrail in a national park to get a better picture and falling to their death, etc.
I'm not saying that this is what happened, but if Caleb was outside waiting for his DD delivery and photographed the bridge and thought it looked cool he may have ventured into the locked field for another shot of something that caught his eye. A lot of explorers will enter places that they know are off-limits because THEY know they aren't going there to steal or vandalize anything so they see their actions as harmless. IF he did this, and IF there was an open manhole cover, I can easily see how he could have fallen into one and quickly been incapacitated by a head injury, fumes or both.
If there may have been an open manhole cover the area should have been better searched, including a search of the piping system. However, IF there was an open manhole cover the cover could have been replaced prior to it being noticed during any searches - possibly by someone not realizing it was significant, not believing it was big enough for an adult to fall into, by a worker who was afraid of getting in trouble if they may have been the one that accidentally left it open, etc.