Both the Russians and The Chinese were infamous for their secrecy during The Cold War years back. I suspect some of that insular paranoia remains part of both cultures even today. The only way we would know about any Russian POWs in the Middle East is if a western prisoner saw them and told us about it, or, more likely, if the Russians wanted us to know. They have nothing to gain by admitting one of their men has been caught, because that would only gain them greater scrutiny from the rest of the world wondering what will the Russians do to rescue their man, and will it be approved of by the ‘World Court’?
I realize what you are saying regarding secrecy by both Russia and China. America was warned regarding Boston by Russian Intelligence. Information sharing is a business. If Intelligence is withholding information can be both good and bad. In this situation if Russia has persons being held by isis would be lucrative for Russia to share that information and corporation. Might work out best to share if Russia is willing. There are currently warnings to Russia of islamic intentions directed at Russia. Russia needs to open up in this instance if they know of some of their people also being held.