If you have an *xref* buffer with absolute Windows filenames, like
~/.emacs.d/init.el
93: server-name (replace-regexp-in-string "\\\\+" "\\" serv t t)
1102: (let ((s (when server-name
1104: (string-match (rx (+ (not (any ?\\))) line-end) server-name)
1105: (upcase (match-string 0 server-name))))))
d:/Devel/emacs/repo/trunk/lisp/erc/erc-backend.el
1820: (pcase-let ((`(,server-name ,server-version)
1823: (setq erc-server-announced-name server-name)
1827: 's004 ?s server-name ?v server-version
and put the cursor in an absolute filename line (like the one d:/Devel/[etc] above), calling `add-log-current-defun' returns the drive letter "d", because it matches a-l-c-d-header-regexp.
The effect is visible when you have which-function-mode enabled, because the function returns non-nil, so which-function does not resort to imenu, and you end with "d" in the mode-line.
I suppose this should be fixed in xref.el, which apparently assumes that file name lines will be either relative or Unix-style (/path/file works correctly, it's just d:/path/file that fails) and the add-log heuristics will always fail.