Corrigendum submitted by my PhD supervisor 1.5 years after I left?
I concur with the comment. This would be a highly unusual way to add new experimental results. Surely if the person was interested in applying or extending your work and had new experimental results they would publish on their own!

I would email them and ask what the contents of the changes are. Ethically, it's up to you if you think that you shouldn't be listed as an author on the corrigendum. There has been discussion about corrigendum authorship in the past, see here. The answer at that link agrees with what I've seen in practice: corrigendum authorship is identical to the original paper authorship even if not everyone worked on it.