Nigel, I don't understand why a monk would assume there's a second red spot when they can see one. All the red spots would appear the day after the abbot was there.

I think multiple monks are implied.

I don't think seeing the reflection is the key to this as I would think it's a no-brainer. They all wash in water, they all drink reflective liquids, maybe eat soup and maybe use shiny utensils. Maybe there's glass or brass or a pond.

I think the key is figuring out why they did it on the same day.