where people swear there weren’t any

It seems pretty common for that to just mean that the person doesn't know what red flags look like.

"We started dating when I was eighteen and he was fifty. He was SO helpful and generous. For instance, he taught me that all my clothes were slutty, made me throw them away, and bought me a whole new wardrobe that he picked out. He also ended all my friendships for me since my friends weren't good enough. Very generous. So there were NO red flags until he killed my dog today."

"She's always been the sweetest, most wonderful woman. Yes, she could get emotional sometimes and maybe yell a little more than I'd like, but we always worked through it and were better than ever after it was over. She didn't mean anything by it when she called me names. It's true that she tried to convince me to have a baby with her after just a month, but that's only because she felt I was the love of her life. There were absolutely zero red flags until she drugged me, jerked me off while I was unconscious, stole my sperm, and impregnated herself against my wishes."

The red flags aren't always that obvious, but they're rarely 100% invisible to people who actually know what they're looking for. I'm sure there are exceptions... just not as many exceptions as some people think.