This question is a perfect example. Nothing has been added or edited. What caused it to get bumped?
As per What does "inactive" mean exactly? which semi-describes inactivity for the Archeologist tag (editing 6 month old inactive questions), activity is any new answers, or edits to the question or answers. Tag edits count for activity, but not for the badge.
That specific question had someone add an answer that should have been a new question. That answer was deleted accordingly.
And it's not just mods who can see that, but anyone that has gained the "Access to Moderation Tools" https://electronics.stackexchange.com/help/privileges/moderator-tools privilege at 10k rep. (Frankly, I wish there was a way to turn viewing deleted answers off, they are annoying).
And as W5V0, unanswered/upvoted questions get bumped by the system to give them more attention.