I'm doubtful of your claim since anything built on church property would be stopped as soon as UNMIK/KFOR saw it or when the church authorities complained. I don't think that the Serbs would have simply let this happen without bringing it to UNMIK. Every time a teen-ager throws a rock and breaks a church window it is reported--it is difficult to believe that a mosque was built over a Serb church and it escaped their attention. Why don't you tell me where this happened--the village or town--and I'll pass this on to the American police element in UNMIK & the U.S. Army element in KFOR so somebody can check it out.
Now, if you're talking about an old site that no longer belongs to the Orthodox Church, perhaps this did occur. But then, that means somebody just used their own property as they themselves saw fit.