Fresh out of the snippets team meeting, Google is testing bolding the URL and/or breadcrumbs in the search result snippets. I doubt the meeting is related to the test, but heck. Either way, Google is constantly testing this type of stuff, I just find it surprising they would test bolding the URL/breadcrumb section.
Sometimes they bold the words you search for in the snippet, the description, title and URL but just to bold the URL and/or breadcrumb section seems odd.
Sergey Alakov posted this screen shot on Twitter of this test:
And Anil posted this screen shot on Twitter of them testing this bold on desktop, not just mobile:
Forum discussion at Twitter.