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....
Learning never ends because we can share what we've learned. And in sharing our collective experiences, we gain deeper understanding of what we learned.