If it's on the employer's machine then you're running their software. Assume it can do anything. On a more realistic note, assuming unmodified software, they could compare the browser history with requests from that machine, and note ones that correspond to holes in the history. I remember way back there was a browser extension that would randomly request pages in the background, to add noise to any attempts to track what sites you were actually viewing. This might have been when many sites used http (no encryption).