XPath 3.0 claims in 2.5.2 that casting to a namespace-sensitive type raises XPTY0117 if the prefix is not in scope.

This is incorrect. F+O, which has the definitive specification for casting, says this is a dynamic error FONS0004.

This must be a dynamic error rather than a type error because it depends on knowledge of the actual supplied value.

XPTY0117 is in fact a different error: it is raised when an untypedAtomic value is supplied to a function that expects a QName (regardless whether the prefix is in scope or not). The error is correctly described in 3.1.5.2 (function conversion rules) but is wrongly described in the error codes appendix.