Decatur Colony was a secret and unknown colony.
It was a research base. The scientists there were sent to research mutations and how to reverse the effects of mutations.
The colony was close enough to the borders of the 4th stage of the test so they could release failed experiments.
During the actually testing for Malencia Vale, she said the thing that scratched and infected her arm, wasn't an animal, it was human. The failed experiments are what Cia encountered. For a long time, Cia thought if a candidate for the tested failed and got redirected they would die. As it turns out, candidates went to Decatur Colony to be tested on.