In 2006, one of Teyla's contacts thought a Planet called Sorth might contain Ancient Technology. In response, McCray flew a Puddle Jumper with his team of seven soldiers and Dr. Radek Zelenka to check it out. They found Radiation spikes and the locals that lived there were showing signs of Radiation poisoning. The locals pointed them to an abandoned factory and the only survivor who had last went there, claimed another group of soldiers had visited prior. They didn't know who they were, and the readings the soldiers received while at the factor had alarmed them so they left. This made McCray antsy, but they continued on.
While at the reactor, McCray's team got caught on cameras and had tripped an alarm. Radek convinced McCray it was fine since the locals already knew they were there. They managed to find abandoned unpowered vehicles and another door that lead further in. While McCray and his team got to work on the door, an armored vehicle with troops ambushed them. Zelenka convinced McCray not to shoot, because they needed to work together and pull their resources. However, it turned out the troops were Genii, and their Sub Commander Argan Loci wanted their surrender. When Zelenka insisted they should work together instead, the Genii fired upon them, causing McCray to have his team fall back, and find better cover. When Zelenka found out the reactor was going to meltdown, he told McCray they needed to evacuate.
Soon after, Zelenka got separated from the group and he tried to contact McCray on the radio but got no response. When he finally got a response it was Argan posing as Tanaka, one of McCray's teammates that Peter had served for serval months with. Radek asked why "William" McCray wasn't responding to him, and Argan lied saying his radio was the only one that still works. Zelenka then pointedly asked him about first names and got a deflection. With this response, Zelenka knew he was talking to the Genii.
After the mission, Zelenka clarifies this in his debrief, that this is because McCray's name wasn't William, it was Peter and the Genii didn't know that.