Yup, it's a proxy.
It ambushed me early this morning with a knife. I have a nasty gash in my shoulder, but I'm still alive for the moment, so I will tentatively count this as a victory. I lost it in a crowded area after about 45 minutes to an hour (I wasn't paying attention to the exact time) of Hide and Seek and Run and Stab. Not fun.
It wasn't wearing a mask, but it was wearing a hood pulled down over its face and closed with a drawstring. Looked like it was a dude, I think. It had a big scary kitchen knife or similar. (The thing certainly cuts flesh well enough.)
I think I will stay up a bit on watch tonight before going to sleep. If I can get the materials, I think I can throw together some simple alarms for the obvious entrances/exits of my current hiding place without too much trouble. Getting the rest of the stuff to do that, now, that might be hard...