Here is how they might discover it:
You stay friends but stop seeing their posts in your Feed.
References:
Think of blocking as deleting a save file. Unblocking creates a new, blank save file. The history is gone.
You unblock them specifically to look at their profile without adding them. This is a bad idea. They will see you viewed their story (if you accidentally click it) or see you in their search history. If you aren't ready to interact, you aren't ready to unblock. what happens when you unblock someone on fb
Messenger has its own bizarre rules.
If someone tagged you in a photo or post before you blocked them, and you unblock them later... nothing happens. That tag is dead. Here is how they might discover it: You
You have three paths forward: