That Mystery String is the Zero-Width Space-- which explains why you can't see it :) --not to be confused with* the Zero-Width Nonbreaking Space which doubles as the Byte Order Mark (%ef%bb%bf). Both have caused trouble for many people over the years: try a quick Forums search and you'll see.
By the time the request reaches your htaccess it will have been decoded. So it's really only one character.
The suggested code you posted is way, way overkill. Does the problem occur only with this specific filename? If so, I smell a bad link somewhere. Or is it a generic issue and you've just illustrated with a random example?
If it is just one file-- or a limited number of files-- first step is to get their names down into the body of the Rule so your server doesn't have to stop and evaluate the conditions for every single request it ever gets for any file of any kind ever. Ever ;)
* Translation: I habitually get them mixed up myself. Sometimes, unfortunately, in forums posts that I can't edit later :(