Gonna say this once and only once:
Derek didn’t know what Parrish was but he knew Kira was a kitsune right off the bat.
Therefore, Parrish can’t be a kitsune. Derek would have known. Even Scott would have recognized something familiar because he’s dating a kitsune.
Key words to remember- “The young ones can’t hide their auras.”
Parrish doesn’t know what he is, so he couldn’t have hidden from them.