INVESTIGATORS probing possible illegal pension fund activities during disgraced state Comptroller Alan Hevesi's tenure are being hampered by the disappearance of "volumes" of sensitive documents, The Post has learned. The documents - providing a paper trail on the investment of tens of millions of dollars in pension-fund assets - disappeared after former Democratic Assemblyman Thomas DiNapoli was named by the Legislature in February to replace Hevesi, who pleaded guilty to misusing state resources and resigned in disgrace in December, a source close to DiNapoli's office said. "This involves actions that occurred on DiNapoli's watch, not on Hevesi's," the source said,...