That is not the sin of scandal on the part of the couple. It could even be the sin of presumption on those asking the “nosy” questions.
Scandal occurs when someone with moral authority over others behaves in a way that destroys the faith within the community. The priest and nun having an affair would be scandal; it suggests to the parish community that the church’s sexual morals don’t matter. Even the unmmarried couple leading the retreat sleeping together might be borderline scandal; they have an influential roll over the formation of the participants
Two participants in a retreat sharing a room, however, in a manner that possibly suggests a sexual relationship is not scandal. The odds are immense that many if not most participants have had sexual encounters outside of marriage, and possibly brought that partner to the retreat. This fact is not unknown to the participants.
The relative moral authority of other participants over each other is simply not so strong that a couple sharing a bathroom but sleeping in separate beds presents a significant foreseeable danger of scandal.
Fornication is the sin of fornication. Even murder is the sin of murder. Scandal is the sin of foreseeably destroying the faith of community through one’s actions; it is a serious sin on its own, separate from the underlying immoral behavior involved or rumored.