After completing the single-player campaign (including side missions, prepper stashes, outposts, and collectibles), I had 4 keys that remained stuck in my inventory as well as one collectable type (the baseball cards) even after turning them in to the NPC.
Below are screenshots (resized a bit to make them smaller) of each one.
As a side note, I had other keys get stuck in my inventory from pick locking the door or chest/locker prior to locating the key to them. When I reverted to a previous saved game and chose not to use the pick lock skill, but instead find and use the key, the key would remove itself from my inventory. I noticed that some doors and chests wouldn't allow the use of the pick lock skill and I'm guessing that would probably resolve most of the issues with some keys getting stuck in the player's inventory (apart from the ones in the screenshots below).
EDIT: Oops, didn't realize I do not have permissions to attach images to this post (only to link to an outside hosting service which I've currently never signed up for one or used one). Until I can get around to doing that, if necessary, I'll just list the items that got stuck in my inventory instead:
- BASEBALL CARDS - Vintage baseball cards to complete George's collection.
- FRED TANNER'S KEY - Opens Fred Tanner's salvaged lockbox.
- KYLE'S KEY - Opens Kyle's Lockbox salvaged from his plane.
- TED AND IAN'S KEY - Opens Ted and Ian's Lockbox of stolen goods.
- WASHROOM KEY - Opens the washroom door in the office at US Auto.
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