The tunnel noises were scripted, I was planning to have the monster appear in the background of other areas of the level, but a number of issues building the AI prevented that. Possibly in future updates but definitely in the final planned commercial release, the monster will be able to move through the tunnels as well. I don't plan on having the monster able to dynamically dig tunnels, but it will be able to crawl through them like it does with vents for example.
Thanks, I've been working on several new mechanics and AI abilities in other projects that I'm planning to bring into the next major update. But since I'll be starting mostly from scratch (the project of this build is too messy and out of date to continue work from) that will likely be in 2022.