We've had a couple of these back door recruitment situation in the last six months.
In house recruiters have to do something to justify their jobs. That "something" is often reducing agency spend. Recruiting an agency's candidates without paying them is one way to do this - provided you don't get caught.
I'm not surprised by these finding, in-house recruiters are being put under increasing pressure and frequently, being ex-recruiters, they'll know the odds of being found out are slim.
I'd recommend, at the very least, linking up with all your candidates so you can see when they moved. If they know they'll get caught, eventually, they'll stop doing it.