Thank you for the offer to try to reproduce the error with my world on your end. I realized that I hadn't tried running the world in SP mode. So hoping to save you the trouble, I tried that this morning on my computer at work. This PC had not run MC before, and once again I used no mods other than Forge. Same thing, without forge I get no missing sign messages. When I enable forge however, I get the messages.
I also tried teleporting to the sign locations, removing and re-adding them to see if that would keep those signs from being reported as missing but I still get messages for the ones I switched out..
Here is the link to my world. I wish you luck, and once again, thanks for taking the time.
https://drive.google.com/file/d/0BxTZeFsVvP8VVGo0VlIyQzkyX0E/edit?usp=sharing