I know there are two extremes: (a) recruiter is a regular employee of hiring company, vs. (z) recruiter works for an unaffiliated placement company.
But i.e. if a recruiter is a temporary contractor with the hiring company, they'd still have an email address from that company, right?
To some extent a contractor who working for the company I know ... I'd still consider that a direct company reaching out to me type situation that I'd be more inclined to respond.
I don't find that it is hidden all that often, but that's just my experience.
I see plenty of recruiters who just work for recruiting firms and don't hide that fact. Anyone who won't immediately tell me who they're recruiting for gets ignored. I'm sure I end up talking to the occasional contractor but you can easily filter out a lot of obvious low-hanging (and rotten) fruit).