It's possible there are infiltrators who are still working on long-term infiltration and haven't yet attempted to add any malicious code anywhere, but the point is that in terms of actual attempts, we've seen a single one and it wasn't even successful despite years of prep.
No, we can't, as that happens a lot via non-serial killers.
A truly successful serial killer is likely one who hides in that noise. No taunting the cops, distributed geographic locations, random methods, avoiding calling cards, and careful not to leave too many traces.
It seems likely that some of the 350k unsolved homicides in the US can be explained this way.
> It's possible there are infiltrators who are still working on long-term infiltration and haven't yet attempted to add any malicious code anywhere…
Or the code's already there, latent, as it would've been in the XZ case, which got discovered by chance and someone very dedicated to looking into a performance glitch.