"I've always suspected one clone escaped the purge."
Ferdinand Chevalier in Orphan Black episode S4E04 "From Instinct to Rational Control"
Ferdinand admits that he suspected that of the seven clones involved in the Helsinki Incident, one escaped the purge. Even though he didn't seem to have known which one, as evidenced by the conversation that follows.
He even admits as much to (Sarah disguised as) Rachel.
"The Helsinki girls were eradicated in 24 hours. 6 clones, 32 collateral."
Ferdinand Chevalier in Orphan Black episode S3E01 "The Weight of This Combination"
Here he talks about six clones having been eradicated, even though there were seven on the list.
So if he strongly suspected that one of the clones escaped the Helsinki purge, why did he not track her down? He seems to be quite thorough in everything else.