Someone copies a source file, maybe you can argue that it’s accidental. Someone copies a source file and strips out the copyright notice, that shows intent.
It takes an argument by a lawyer in front of a trier if fact before stripping copyright from source files shows anything. Irrespective of details, the wherewithal to lawyer up is mostly what matters because “fuck you sue me” is a sound legal strategy in so many cases. Or to put it another way, unless you have a lawyer on retainer the odds of bring a ne’erdowell to heel are low and one on retainer is probably saying it’s not worth it if they’re worth their salt. YMMV
Someone copies a source file, maybe you can argue that it’s accidental. Someone copies a source file and strips out the copyright notice, that shows intent.