"Occupying the field" refers to a situation in where federal regulation of a matter is "so pervasive as to make reasonable the inference that Congress left no room for the States to supplement it.” Rice v. Santa Fe Elevator Corp., 331 U.S. 218, 230 (1947). In other words, if the federal government has so comprehensively regulated an industry or product or activity, the states may not impose their own regulations.
That concept would not apply to the records of the Capitol Police, for a few reasons. First, I don't know of any state laws that purport to govern access to records held by the Capitol Police. Even if such a law did exist, FOIA would not "occupy the field," as it governs only records held by certain components of the executive department, 5 U.S.C. 552(f) .
The common-law right of access would likely not apply, as it is rarely applied to anything other than records held by the judiciary.