Does Barbara Boxer have an investigation on police officers and their unfair treatment of alleged offenders in Georgia's Cobb County?

No. Boxer is a California member of federal Congress and has no authority over local police in CAlifornia, Georgia or anywhere else.
The ACLU has repeatedly alleged -without evidence - that it is illegal for CObb County law enforcement to comply with a federal program that allows local cops to notify ICE when illegal aliens are arrested.

Were Senator Boxer involved, she would certainly congratulate the Cobb County sheriff for complying with federal-local cooperation programs and eliminating illegal alien felons from the USA.