buffer-stale--default-function is a compiled Lisp function in `files.el'.

(buffer-stale--default-function &optional NOCONFIRM)

Default function to use for `buffer-stale-function'.
This function ignores its argument.
This returns non-nil if the current buffer is visiting a readable file
whose modification time does not match that of the buffer.

This function only handles buffers that are visiting files.
Non-file buffers need a custom function