During the daytime, one of the Med Center Air helos is based over at Rock Hill Airport. At sunset, the helo is flown back to a hanger at the airport in Monroe, then brought back the next morning.
(The big advantage of keeping one of the helos away from downtown Charlotte and Charlotte-Douglas Airport is that they can often respond to a flight request faster, since there are less restrictions on the air space for flying over Rock Hill.)
If the helicopters you saw were related to Med Center Air's program, the overflights may have been related to either finding a new landing zone to pickup critical care patients (often accident victims), when transport time to a trauma center is important.
The helicopter could have also been related to a SLED or National Guard drug detection operation. The cargo bay door might be open for video surveillance, or to use IR detection on a possible grow house.
BTW - Med Center Air switched to new Eurocopter EC135 helos in February. Last time I saw the one based in Rock Hill, it was still not decalled in the CMC / CHS real green and white colors. Here's a link to some photos of the new med helos they've gone to ...