They are very cunning little beasties, they can produce a thread and abseil down, or just drop onto the ground. I find it is worth going back every day for two or three days because however carefully you squish you miss some.
They do not seem to like my nasturiums very much.
Even though you did not see them there must be at least two!
Good idea to pursuade other plot holders to squish as many as possible to reduce attack next year. It is another frustration on untended plots that they are also breeding butterflies.