"Yet we were releasing, in the previous 12 months, about 150 directly from closed conditions not having been tested under open conditions," he said.
"I don't know whether this is right or wrong, what I am saying is we need some evidence and some help in this matter in terms of risk."
He told MSPs some steps have been taken to rehabilitate offenders, including giving them their own unit at Glenochil prison near Stirling, but concerns remain about how they are handled.
"What I really worry about is that at some stage offenders are going to be released from those conditions directly back to the community," he concluded.
A major report on Scottish policing, published yesterday, showed that 3,946 sex offenders are being monitored in the community by the authorities.
This was a slight increase on the previous year and 210 more than two years previously. The Tories reiterated their call for sex offenders to be subjected to satellite tracking.
John Lamont, the party's chief whip, said: "We also think if the Scottish Government dropped its automatic early release, as it has promised to do on more than one occasion, this would allow such offenders to serve fuller sentences and have more chance of being rehabilitated."
Kenny MacAskill, the Scottish Justice Minister, recently announced that criminals are to be tracked by satellite, a move that if successful could be extended to sex offenders.
However, he caused consternation by handing the £13 million contract to G4S, the firm at the centre of the London Olympics security debacle.
A Scottish Prison Service spokesman said the organisation is considering how to move more sex offenders to open prisons "while ensuring public safety is maintained".
A Scottish Government spokesman said: "The monitoring of sex offenders in Scotland is tougher than ever before and many are required to register for long periods of time, with some registering for life."