Well guests can see who liked a certain post so that's normal, the "not_found" error means that they've encountered an error screen, in this case "Sorry, this section isn't available at this time." but it's not printing the error message because the Errors language file isn't loaded at the time.
I posted a fix to GitHub that'll fix it.
https://github.com/SimpleMachines/SMF2.1/pull/5756It's probably a bot though, since they accessed the page with an incorrectly formatted URL, something like this I think:
hxxp://www.yourforum.com/index.php?action=likes%3Bsa=view%3Bltype=msg%3Blike=509%blablabla=d89051xsdfa39000dsdf7cb6bcc
Thanks for the report
