@stephenhedley, I seem to recall you can poison her, set fire to her, or cast a miracle like tranquil walk of peace, or fire an arrow into her, all of which can help you to locate her when she's invisible. Also I think you can see her footprints in the snow, which may be of help.
I'd also like to post that on my final run to the final boss in my latest playthrough, I stumbled upon my first ever Vagrant. I had no clue what the hell it was, I was just laughing at it thinking it looked like a turkey (from a distance), then it almost killed me in one shot with a volley of needles, the wee bastard! So nice to have a game still hold surprises after you think you've seen everything it has to offer.
Don't point that beard at me, it might go off!