I believe in HTML that may be the · character, or something similar (I haven't coded HTML in awhile now to remember). Whatever it is, it is outside the standard range of ASCII characters. With the dot, we could not find the thread with a thread title search. We edited the thread title to replace the character with a hyphen, and the thread now appears in search.
I guess on a Mac, the Command-8 combination will insert that character. In Windows ALT+0149 on the keypad will insert it. This could be more a bug in ElasticSearch than in XF, though (and regular readers know how I feel about ES ).