Well, to all who still miss Sigurd, I think I found a post that may shine some light on the situation.
I found a not-so-old post from an anonymous user on a old-fashioned site that had some...less than friendly stuff to say about Sigurd. oweeh6jcqx3aoey2.onion.link/v/…
Reading inbetween the lines, it sounds like Sigurd was copying the style, and even some of the ideas, of a japanese artist named Konpeto. Their content can be found on most Booru sites, and they have their own Twitter as well. Out of respect for their privacy I won't be posting it, but if you do some searching on Danbooru and Gelbooru you'll find the links quickly enough.
If what this asshat says is true (and I desparately don't want to believe it) Sigurd is one of the most skilled art thieves this generation has ever seen. It could very well explain why he just up and disappeared. If someone caught onto him, it makes sense that he would stop.