auth-source-debug is a variable defined in `auth-source.el'.
Its value is

Whether auth-source should log debug messages.

If the value is nil, debug messages are not logged.

If the value is t, debug messages are logged with `message'. In
that case, your authentication data will be in the clear (except
for passwords).

If the value is a function, debug messages are logged by calling
that function using the same arguments as `message'.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 23.2 of Emacs.