The searchmode param is usually added by the Search Web Part (With or without results). Check the settings there if you need it.
Question though: does it actually break functionality? Or is it incidental in the EventLog.
If you're not causing it yourself there are always going to be some errors now and then. Could be wrong encoded by some obscure browser or external robot search indexer... someone testing your site for vulnerabilities or sometimes 'bits' just get lost along the way.
So unless it's flooding your event log, actually breaking functionality or real users report getting an error (often) I wouldn't spend to much time trying to fix it.