Piotr hadn't wanted to leave Illyana's side, but he realized that she was not only 16 years old, but she was a very capable 16. She was perhaps more socialized and more used to human interaction than he had been at 16, but he felt that she was just as capable of taking care of herself as he had been at that age. If her gun and willingness to pull it on somebody who claimed to be her brother had any indication. Also how well she had accepted their current situation.
So eventually he had, and he'd made his way to try to find other people. Cas seemed the most difficult to track down, until Piotr realized that they were likely looking for one another, and circling each other like dogs on the hunt. He didn't think that Cas would cease his search, so Piotr made the hard decision to be the one who stayed put. It went against his nature and his training, but the logic couldn't be argued with.
And all of it was proven when Cas spotted him and made his approach. Piotr noted the bit of a smile, a monumental thing when it came to men like them, and returned it.
"Cas. It is good to see you, my friend." There was a feeling that there should be something more there. A bigger greeting. Piotr imagined that others who were friends might embrace. Instead, there was an awkward pause.