[newcats]

Hawkings postulates matter disppearing from black holes ... But doesn't ask "where does it go?"
Matter can be transformed into energy can't it? And energy can take many forms, most of them invisible to the human eye.

[Robert A Cook PE]

Hawkings actually goes much deeper than that, although his thoughts begin with the mass-energy equivilence and quantum "changes" between different indeterminate states of mass.

Basically, he theorizes that, at the center of a black hole, mass - being equal to energy, can "leave" the black hole.