The first two explanations might leave some kind of trace—debris, cross-trail of some theoretical attacking force, radioactivity, whatever. The third would at least show up as a redirected Bussard trail. But to detect any of those alternatives, I’d have to be crawling along at 5% of C. That would require 320 years to completely scan Bender’s projected path. Of course, if I found something, I wouldn’t do the whole route, but it would still be a whole lot of not very much for a long time.