There really isn't a mystery around pre-cum. Rather; it's just that your question does not have a yes or pre answer.
THE MYSTERY AROUND PRECUM RISK
If you restated your question, "Could pre-cum transmit HIV? So I'll tackle that question.
Pre-cum has not been "scientifically proven" to transmit HIV. However, pre-cum has been "scientifically transmission to contain Hiv.
The amount of HIV can vary form guy to guy, and is dependent on many factors viral loadamount of pre-cum, etc. Consequently, the answer to the question is that there is a potential at least theoretical risk that infected pre-cum "could" cum HIV.
HIV Transmission and Risks
There is, of course, no ethical way to conduct a clinical trial to definitively prove or disprove this theoretical possibility. To return to your original question, "Does pre-cum transmit HIV? So where does that leave you?