Hi Eve,
I don't think that is a virus. My renanthera plants do that when they are in heat shock so they decide to loose a few leaves. And your plant has renanthera parentage in it. So I always try to avoid that sort of things. Another possibility is that leave might have been attacked by a fungus and the plant decides to rid itself of the infection by killing that leave! So remove the yellow leaves as soon as you can do so and keep the plant at a more even/temperate temperatures (like in the 70s during the day and upper 60s at night).
Cheers. Hoa.