I have worked for a company which accidentally shipped unlicensed assets that had been used during internal development. These assets were not ever visible in the shipped game, but they had been overlooked and were accidentally still present amongst the game's data files, and were uncovered and reported upon by the media, and this caused some major legal/licensing issues for the company.
Using assets that you don't have the rights to use -- even exclusively for internal-only development -- is very dangerous. It's far too easy to forget to remove those assets before shipping. It's much, much safer to only use assets that you can legally ship, just in case something gets overlooked in the rush to release.