At the Easter weekend my trap was set in Derwenlas on our patio table adjacent to a patio umbrella which was contained in a green canvas weatherproof cover. The night temperature was quite low and there was some dew on the ground. In the early morning I found this image on the canvas cover.
Sorry about the "rings"
The most likely culprit judging by my catch in the trap was a Puss Moth or a Great Prominent.
An hour later there was no trace of my "ghost" moth.
The cover didn't appear to be wet. I assumed it may be some temperature differential or condensation effect but if any of the scientific minds of Montgomeryshire can offer a rational explanation please do let me know!