If two people have mutually agreed to be in a polyamorous relationship, then having a relationship with another person isn’t “shady” at all. Although they may have rules about openness and honesty that are being violated. It sounds like at least one person in the relationship isn’t actually comfortable with the polyamory, which is something they may want to discuss. However, if the relationship truly is polyamorous, then snooping would be a violation of trust and privacy. It is possible for both people to be at fault here: Person A for snooping and Person B for cheating (if the relationship isn’t an open one). Both need to take responsibility for their actions if they want the relationship to move past this.