.find() uses Sizzle, which appears to always return an empty array when a document fragment is used:
In the test case, when this bit of Sizzle code is executed,
context is the document fragment, which has a
nodeType of 11, therefore zero results are returned.
Looking further down in sizzle.js, it appears document fragments are excluded because they don't have some methods used by the shortcuts, such as
getElementById (line 250),
getElementsByClassName (line 278), and, what would probably be used in the given test case if this were not a document fragment,
getElementsByTagName (line 274). There could be other methods Sizzle expects from
context that document fragments do not include.