TCP/IP ports which are not in regular use (quiescent ports) can show surges in activity for several reasons. Two examples include the discovery of a vulnerability in an unused (but still present) network service or a new backdoor which runs on an unassigned or obsolete port. Identifying this anomalous activity can be a challenge, however, due to the ever-present background of vertical scanning, which can show substantial peak activity. It is, however, possible to separate port-specific activity from this background by recognizing that the activity due to vertical scanning results in strong correlations between port-specific flow counts. We introduce a method for detecting onset of anomalous port-specific activity by recognizing deviation from correlated activity.