Is it totally sealed? Have you got rabbit fencing a foot below the surface all the way round?
It is quite odd, a mouse would usually leave droppings, and to make lots of small nibbles from orchids would surely not be what they want to eat.
but i cannot think of creatur with such a bite size. Mice can sneak in pretty much anywhere mind.
How clear is your greenhouse? I would remove everything but the orchids and have a thorough check in bags of compost etc.
Put down some traps, you may well catch something.
have you got a good sound recorder? It sounds silly, but if you have a good one, and can upload it, with abit of software you can quickly look through it for possible loud sounds during the night.
Depends how quiet your area is mind.
CCTV maybe help too
