Recently I've noticed that just above the web page logo, strange glyphs appeared. What might be the cause? How to get rid of these.
If there is anything printed above the logo, then in most cases it's caused by some errors in PHP. So somewhere in your theme or in one of plugins there is some bad code that is causing this.
And these characters suggest that they're some badly converted UTF characters or some other characters that are not supported by your font.
From my experience - such problem occurs when site gets infected and malicious code is working not entirely how its author wanted it to...
So I would check your theme and then all other files of that site if they were modified and if they contain some code that doesn't look nice.