While conducting a covert penetration test on a UNIX-based infrastructure, the tester decides to bypass...

While conducting a covert penetration test on a UNIX-based infrastructure, the tester decides to bypass intrusion detection systems by sending specially crafted TCP packets with an unusual set of flags enabled. These packets do not initiate or complete any TCP handshake. During the scan, the tester notices that when certain ports are probed, there is no response from the target, but for others, a TCP RST (reset) packet is received. The tester notes that this behavior consistently aligns with open and closed ports. Based on these observations, which scanning technique is most likely being used?