Baltimore City (B/C) and Anne Arundel (A/A), Baltimore (B/A), Howard (H/C), Montgomery (M/C) and Prince George's (P/G) counties all have independent police departments that are assigned their own plate series.
A/A 1590: Anne Arundel County Police
B/A 2055: Baltimore County Police
H/C 1103: Howard County Police
M/C 0936: Montgomery County Police
P/G 3812: Prince George's County
P/P 0288: Maryland National Capital Park Police, Prince George's County (There is also a Montgomery County version of the National Capital Park Police plate, also with P/P prefix.)
S/P 0053: Baltimore City School Police
Tom Castelli informs us that other counties not listed here have a county sheriff's office that performs the same functions as an independent county police department. These departments use similar plates in the S/A 1234 series with "[county name] Co." screened below the serial. Cities and other municipalities that have independent police departments (e.g. Laurel, Westminster) use similar plates in the M/G 1234 series with the name of the municipality screened below the serial, although the L/G series is used by some towns.
Municipal series:
M/G 2376: Westminster
Sheriff series:
S/A 1800 series, S/A 3900 series (Baltimore City)
S/A 1932 (Worcester County)
S/A 2400 series (Baltimore County)
S/A 4720 (Wicomico County)
S/A 5913 (Carroll County)
University of Maryland Campus Police: C/P 0105
From Tom Castelli: It was the standard police type, with a caption of "University of Maryland." This was on a UMBC campus police car on the UMBC campus; I assume this type and prefix is used across all UMd campuses for their campus police cars. (Also assuming C/P stands for Campus Police.)
Maryland Department of Labor and Licensing Police: L/L0003
Reported by Jeff Ellis