In the grid view, hitting "J" cycles through different ways of viewing (or not viewing) the information set up in the "Library View Options" window ("Command-J"). Perhaps you inadvertently hit "J" at some point while in the grid view.
I don't know how (or even whether) developing would effect this. I tried a quick test starting from the thumbnails having no information, developing an image, and then viewing once again in grid, but the icons you refer to did not reappear. Hitting "J" in the develop module turns the clipped warnings on and off.