[FallingAlice]

Have you talked to her about it? Either casually or in the context of a formal "exit interview?" Not to say that she'll be honest with you. She probably won't. But it's worth a try.

From your perspective it was a great fit and she was doing well. But it's rare for someone (especially a bright woman in math and science) to do what she's done without some reason. Most of us don't leave employment that we really like.

Maybe from her perspective she didn't feel as though she fit in. Maybe there were things in the organizational culture that she found distasteful. Or maybe she felt that she was low-balled during salary negotiations and pulled a T.O. in order to get a whole lot more money elsewhere.

I don't what your company does. I don't mean to be sexist and I'm trying not to jump to conclusions but is your firm very male dominated? I've had friends and have myself worked in intensely male-dominated environments that we all found personally repellant. In these cases, most women pretend to get along, bite their tongues and find a way to leave as soon as possible. I'm not talking here about an environment of harrassment and overt discrimination, but one of not-so-subtle hostility and/or indifference. This is particularly true in science and engineering fields.

If she has a particularly savvy coping style then she could well leave you with the impression that she was comfortable with your company all the while hating it.

Of course, it may not be a gender thing. It may be how interesting the work is ... what she began to perceive her career trajectory might be ... her opinion of her supervisor... lifestyle and general corporate culture issues ... or ... again ... money. Regardless, I would look at this as a labor market issue (not so much an ethical issue) and I would try to find out why she bolted so fast.

In the end, the important question for you is employee recruitment and retention. In the long run, the answer to that question is what's valuable to your company.