Whenever I see an em dash (—), I suspect the entire text was written by an AI.
replies(7):
AI almost certainly picked it up mainly from typeset documents, like PDF papers.
It's also possible that some models have a tokenizing rule for recognizing faked-out em-dashes made of hyphens and turning them into real em-dash tokens.
On my own (long abandoned) blog, about 20% of (public) posts seem to contain an em dash: https://shreevatsa.wordpress.com/?s=%E2%80%94 (going by 4 pages of search results for the em dash vs 21 pages in total).