Something like that could have damaged the retention brackets too.
I had this on my last car. Colleague reversed into it in the car park and the hidden damage was surprising.
		
		
	 
Years ago I stopped at a red light in Pelham Street SW7, facing Sloane Ave SW3 in Chelsea, in my Black Cab, I had no job in the back.
A woman driving a 5 series BMW ran into my rear bumper, and when we looked, my bumper was intact, as was her front bumper, my number plate was higher up, on the boot lid, also untouched.
Not unnaturally, she was full of, "No big deal, neither of us has sustained damage, forget it eh, sorry and all that."
I wouldn't have it, I said, "You did hit me rather hard, let me have it checked out under the boot, and I'll call you."
She wasn't happy, but she could hardly argue, so we exchanged details, she gave me her phone number, and we drove away.
When the guy who did my servicing put the taxi up on the lift, the impact had pushed some strut under the boot, into the fuel tank, not rupturing it, but causing a fair sized indentation.
Andy, the garage proprietor, said, "While that's nothing to worry about it now, when it has its annual TFL inspection, to be plated as a London taxi, they will not pass it in that condition, it means a new tank, plus struts."
To say the woman was not happy was an understatement, she started by accusing me of lying, and taking advantage of the situation and her, because she was a woman, and ended by demanding a letter proving that what I'd said was true.
I had Andy's office girl type the letter up for her, and enclosed it with the estimate, which I said I said that I'd be happy to accept cash for, or go through insurance, whatever she was happy with, and I'd provide her with a copy of the bill, to prove that I'd had the work done, she opted for the cash route, and Andy added £25 to his invoice to cover the letter his girl had typed, naturally I hadn't paid for the letter, he'd done that under the old pals act.