Well, I thought he was holding Owl's towel, but it's more likely Beetle intended to bathe with them from the start.
His reaction to the scene on screen made me think he had no idea what Owl and Prismo were doing there (and maybe he wasn't invited to participate), but perhaps he was just distracted getting a towel for himself and missed part of the conversation. The way he positioned himself didn't help either; he looked like a butler.
For a moment, I felt offended by him. I was worried; I felt like they were hurting his ego.
But it wasn't any of that.
I keep seeing the God Auditor; I almost couldn't help it.
Although I'm now very glad that they share the jacuzzi, even the idea of Scarab holding a towel for Prismo "just in case he needed it" seemed sweet to me.