Was the "groove" like the one I found this morning on my "Allons-y, Dr.X"? This is the tomato after it was all cleaned up:
If so, there were frass (bug poop) in the groove when I found it, and a tomato fruit worm was halfway burrowing under the stem-end/calyx at the end of the groove. I dispatched it by poking it out with the tip of my pruners and then cutting it in half....