According to Nestov, Fenhill was a nasty criminal who 'had a bad habit of making Orsians disappear' - suggesting that Fenhill had a xenophobic hatred of them. This hatred resulted in him murdering many before eventually been caught and sent to SAR TOP.
After escaping, he took on the human form of a woman called Cheryl Vandeross. When Cole began to track Fenhill (been away from the Watchfire for 3 days), he eventually learned of the identity of Fenhill's human form - but Fenhill had already escaped to California. Investigating further, however, Cole quit his search after finding a video tape that's Cheryl's boyfriend had recorded (having been at the train station on the same day that the prisoners came to Earth).
Returning to the Watchfire, and after explaining to Mel and Nestov what he'd been doing during his time away, Cole played the video through his computer - revealing that Cheryl's boyfriend had recorded more of the prisoners who came in on that day than what the ATM machine did (that Cole got footage from earlier on).
It's likely that Cole either captured Fenhill at a later point during his hunts, or Fenhill was captured with the remaining prisoners when Cole found a way to capture all the fugutives' life forces at once.
Tracker (TV Series) - Episode 16: Dark Road Home (Mentioned Only)