Here is an interesting case: A company provides a free (MPL-licensed) library (called JointJS), and furthermore a number of proprietary extensions (called rappid).
On their web server they ship a file that combines code from both parts of the library at http://jointjs.com/js/joint.all.min.js (gist for historical reference), as it is quite common for JavaScript libraries. But note that this file contains only a copyright or license statement for the free JointJS part (and a few other free licenses for third-party tools). So the structure of the file is:
/*! JointJS v0.9.3 - JavaScript diagramming library 2015-06-26
This Source Code Form is subject to the terms of the Mozilla Public
License, v. 2.0. If a copy of the MPL was not distributed with this
file, You can obtain one at http://mozilla.org/MPL/2.0/.
*/
[Similar license header for first third-party library]
[first third party library]
[Similar license header for second third-party library]
[second third party library]
[JointJS code (likely intended to be released under the MPL]
[rappid code (when separately distributed, not released under the MPL]
So assuming I were pointed to that file only (as you are now), there is no way of knowing that parts of that code are not intended to be licensed under the MPL license as stated. And given that fact that the actual copyright owner did that, does this imply that all parts of that file can now be used under the terms of the JointJS part itself?