Since we're in the world of statistics, which, by definition, includes "randomness and uncertainty modeled by probability theory" (source: Wikipedia) and what Avinash calls the puzzles and mysteries, I would tend to put this kind of traffic in the "anomalies" category. We more than often face mysteries, and trying to explain and achieve perfect results (like in a puzzle) is usually not reasonable.
The question bares down to the overall significance of this traffic and the "cost" (in terms of detection, human time, computing power, analysis, etc.) to exclude it from our results. If it's not significant (say, less than 5%? 2%?), I would just not worry with it. We usually look for trends and spot "anomalies". If we can explain those abnormalities, be it strong emarketing results (good!) or unexpected crawlers (less good!), I think we're in a pretty good position.