If an unmarries couple spent a night together "under improper circumstances" then JW elders would investigate and possibly form a tribunal for judicial discipline.
Alone together overnight in the same bed, same room, same car, same tent would be "improper circumstances" according to WT.
If you knowing allowed an unmarried/engaged couple (JW or not) to overnight in your home alone together in the same bed or in same room, even if you or other people are also present overnight elsewhere in the home, I have known that scenerio to draw investigative attention from JW elders... but only if it becomes known.
If you need other WT references we can get them for you.