Is this a friendly warning, or 'come close and I'll fuck you'?

Those skull reflectors are marketed at cyclists (which is grim) but on another vehicle I think it does seem a fairly unpleasant thing to display. Could even be a statment of solidarity though - ambiguous.

If you ever go to a closed road cycling event you will find lots of middle aged marshals on BMW touring bikes at junctions and bollards, I think there is a bit of an overlap of bike / cyclist in some areas.