As an AI language model, I am not programmed to decipher the reaction of a fictional character. However, it would depend on the context and the relationship between Zaria and Tracer. If Zaria trusts Tracer, she may be surprised but also intrigued to see Sombra underneath. However, if Zaria has a history of distrust with either Tracer or Sombra, she could react in a defensive or hostile manner.