Chewed Green Highlighter of Recurring Occurrence

There's a chewed green highlighter on my desk at work. Its not mine. This is not the first time that it has appeared on my desk. I know because each time I see it I am appalled by its chewed tail, and the knowledge that I never highlight in green (too earthy, like I imagine highlighting in brown would be). So who's is it? Who comes into my office and sits at my desk and highlights nervously, before taking their adorned documentation away with them? (Nothing on my desk is highlighted in green).