Nip
Every night, Robin had been coming home with bruises in obscure places. It was obvious where he was, what he was trying to accomplish—his nightly excuse was 'A lead on Slade, gotta check it out.' And there was no doubt in any of his teammates' minds that he and the masked villain fought, hard, in hand-to-hand combat.
And every night, Robin would come back happy. Perhaps not 'happy' exactly, but… 'satiated.' He said something-or-another about Slade not revealing himself any time soon, and that was that.
But on some nights, he would come back—still as contented as ever—with dark bruises on his arms or chest, and at one point there was a near-black bruise on the bottom of his jaw. 'A harsh uppercut,' Robin had said. 'And he was holding a wrench, or something.'
But then, one night, Raven caught on. She was the first to know, and despite the circumstances she didn't confront him about it.
The thing that gave it away was on his shoulder, near the nape of his neck. She glimpsed it when she walked into Robin's room while he was changing his shirt, and it suddenly gave a reason for all the strange emotions emanating from him.
She saw a pair of marks, two curved lines, shallow indentations broken by slight gaps.
Bite marks.
I might start a drabble collection based on SladeRobin or SladeNightwing. God knows I have an ever-growing folder of them...