They get caught

The night continued like that for about half an hour. With Jaune and Pyrrha fighting and flirting, and Pyrrha stripping whenever Jaune's sword made contact.

Weiss quickly noted that Pyrrha wasn't trying to advance on Jaune at all. No, she was only working defensively. Evidently, this exercise was meant for Jaune to work on offense. And, clearly, it was working. Weiss had no way of knowing how long they'd been doing this. But, as she noted before, Jaune had improved quite a bit. To the point where Weiss wasn't sure if she'd be able to hold out against him, in a one-on-one fight. At the very least, Jaune would give her cause for concern...

It was after Jaune made contact with Pyrrha's headpiece, leaving her with no armor above the waist save for her shield, that it happened. As Pyrrha was walking over to store her lost article with the rest of them, Pyrrha just happened to turn towards the streaker's hiding spot and locked eyes with Weiss. And if she saw Weiss, doubtless she saw Blake as well.

Weiss's heart began pounding in her ears. It became difficult to tell how long she had been looking, as one second stretched out into eternity. The humiliation of being caught naked, on the roof of all places! There was no good reason for the two of them to be up here, even with clothes on!

Oh gods oh gods oh gods oh gods oh gods, Weiss thought as she feared what Pyrrha would do in response to seeing this.