The licensing of a copyrighted work involves:
- a set of permissions (the license) and
- an association between a particular work and the license
In this case you clearly have item #1 (the GPL is certainly a license) but the scope (indeed, the existence) of item #2 is unclear at best. In this case, it seems clear to me that there is no real case for seeing an implicit "or any later version" clause, since it isn't even obvious that the files (or which particular files) are licensed under the version of the GPL that is being provided.
The salient legal question here is: if an author released code alongside a license like you describe, would they prevail in suing a downstream distributor/modifier who acted in a good-faith belief that one or more of the files were licensed under the GPL, based on the license's inclusion alongside the code? For the case where the downstream user claims the work was licensed under exactly GPL version X, it sounds like a pretty strong defense, and the suing author will fail. For the case where the downstream user claims the work was licensed under some later version as well, it sounds like a pretty weak defense, and the author will probably win.
A related interesting case is the inverse of this, in which a copyright holder declares that a work is licensed under the GNU GPL without a version number. In that case, you may choose any version of the GPL, since every version of the GPL states, "If the Program does not specify a version number of the GNU General Public License, you may choose any version ever published by the Free Software Foundation." I can't say how a no-version GPL declaration within the program would interact with the program being placed along side a specific GPL license, though.