I did some research and added some comments with an issue link:
Since the Activity will show whatever is in the comments you could search in that manner if you know there is a comment with that link but there isn't a search box per-se that you can search from.
I have not seen a feature request for this so I would also advise that you go to JAC and create a Suggestion request so we can get that feature added into JIRA. If you do create the Suggestion please post the link here so the Community can vote on it and add impact.
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