[Solve the mystery] Old Facebook Ad Tracking link still drives traffic

Hello there!
I have a mystery to solve for you that is currently happening in my Google Analytics data.
Here is the story:
The website I manage was added this week to GA to look closer at incoming traffic and conversions. After running for a few days, I noticed some visitors came from a certain redirect link and actually converted afterwards.
Now, I asked the previous marketing managers about this link and it turned out to be a redirect from old Facebook ads these managers tried out last year. Since the ads were not that successful, they shut them down after several days.
As I looked into the Facebook page, I found the posts from those ads that were shared and deleted them. (I would have changed the link, but apparently thats not possible)
But still, traffic is coming in.
Then I checked the page for more of these links and didnt find anything, except the Action button on the page and changed it.
But still, traffic is coming in.
So I tried researching about the redirect page itself. But going to the domain, I only find a 404 error message. Plus, as deleted the ads earlier, I saved the redirect link and tested it. Of course it's still working.
Unfortunately, the previous manager does not remember anymore how he got that link or where else it could have been placed, so its hard to track it down now.
But as it still drives traffic and conversions to our site, it would be a waste not to get behind it.
What do you think? Is this case just too obvious that a SEO newbie like me just doesnt see it or is it a real mystery?
Looking forward to your thoughts!