Or, more to the point, not one single person has any recourse, whether they're an evil serial killer/rapist or not.

But, I'm trying to think about a situation where a secret identity might leave behind trace evidence that could be linked to a readily identifiable relative's samples, thus unmasking... say Spiderman.

I guess, any situation involving trespassing or physical presence under other circumstances, where a person might leave behind a single hair follicle, including the root of the strand.

Definitely. As the number of sequenced individuals grows, it'll become that much harder to hide from a familial genetic search.