Some recruiters will actually recruit from their own clients. Believe it or not there are some municipal recruiters who have so many clients and so few candidates that they literally have no one left to recruit from.
Against all reasonable expectations to the contrary, many of these companies will, in fact, attempt to recruit from your staff even while they are under contract recruiting for you. This is a lot like letting the fox into the Hen house. In many cases even after they had been paid to help you find someone they will unashamedly go after the brightest and best of your staff. Not long ago one CAO told me that after awarding a search for a Director of Planning, the firm he had hired, made three separate attempts to recruit him away to a different city.
Always ask your recruiter if you are protected from this kind of activity. Ask them to guarantee this and have them spell out the length of time that guarantee will be in place.