Why is a post cache file cleared when a post is restored from the trash?
If you restore a Post/Page from the Trash and you see a message about a cache file being cleared for that Post/Page ID, you likely have 404 Request Caching enabled (see WordPress Dashboard → Comet Cache → Plugin Options → 404 Requests).
When 404 Request Caching is enabled, a cache file link to the 404 Page cache file is created whenever someone visits a post URL that returns a 404 page.
What this means is that if you have a post in the Trash and someone tries to visit that Post, Comet Cache will create a 404 Cache File link as if it were creating a Post Cache file for that post. When you Restore the Post/Page from the Trash, Comet Cache deletes this cache file link to the 404 Page and that's the reason you see a message about a cache file being cleared.