A few random thoughts:
Southern Ontario is full of heavily forested land. A lot of these venues are surrounded by bush, I would imagine. Maybe more of these things, connected to the Games or not?
I like the cadaver dog idea. It would be prudent to check the tunnel and surrounding land, just in case.
It appears from the tone of the original press conference that the tunnel was not booby-trapped, which is good. (But then again, no traps make it appear more innocuous, and that is good for the criminal, if this is a crime.).
I think we are playing with semantics when we describe it as a "tunnel" vs. a "chamber", or whatever. The point is that we don't know whether it was going to be extended outward any further, or just worked on in some other way on the inside, to completion. It had elements of both. What I find troubling is that a long "hallway" is a good way of trapping intruders who drop down from the entrance, while you are in the "pod" portion at the end, possibly with a weapon.
I had read that police found evidence that people were hanging out at the tunnel, eating and drinking. So, people were there. What exactly they were thinking or up to, is a mystery.
Theft is never a good thing. The fact that the beams were possibly stolen from the Rexall Centre is not a good sign. It may be just petty crime, but taken with everything else, it is not good. JMHO.