My daughter also would get popped for biting, which she did often and viciously. For that I would slap her lips. It didn't work, though. She didn't quit biting until about a year ago (she's almost 8 now).
Guess what we did? You'll die, it's barbaric. She bit her brother on the back, hard enough to leave marks that lasted (a few times). My second boy, who is in his 20s, finally had had enough and the last time she did it, he took her aside, called the bitten one over, and said, "Now it's your turn" and held out her arm.
Lol. We ended up doing that twice only, and that was the end of the biting forever. I wish I'd thought of it sooner. It wouldn't work for a tiny baby, but it worked for the kids.
And the one we allowed to bite her is soft-hearted, he didn't (and we knew he wouldn't) bite her hard. But she was appalled.
The thing is, you just don't know. I know there are abusive people out there, but there is no way to know second and third hand whether this was abuse.
I've also had a Walmart checker grab my daughter (same daughter) from me and plop her in the cart. I had a broken foot, I was holding onto another squirming toddler, and was paying for my groceries. I sort of had my daughter pinned against the counter because she would take off and run blindly from me and we were just steps from the doors to the busy parking lot. This girl has been faster than me from the time she could walk, she's like a deer, and she literally would just race away and be gone in a split second.
The checker saw me pinning her and holding onto her hood and thought I was abusive. But she didn't know my girl. I wasn't punishing her, even, I was restraining her. This is the same girl who bit her brother, kicked me (fairly regularly until she was 6) and would throw herself straight back, stiff as a board, when she was upset (full body slam, backwards).
Ah, those were the days.
My primary concern was keeping her alive. I didn't worry too much about how it looked to people.
And she is the absolute darling of the family; my only girl! You've never seen a more beautiful child, or a more sparkly, smart and cheerful one. Just had some trouble with that impulse control until the last year or so....