Holes/skeletonized orchid leaves HELP!

[Greenthumb Mern]

I have three orchids in my tiny beginner collection, a Phal, a Dendrobium hybrid, and something that I rescued from Walmart a few years ago that might also be a sort of Dendrobium, I don't know.
All three have something invisible (I haven't seen it yet) eating the leaves when my back is turned. It's still happening on the new growth even though I've brought them inside for winter.
There are small holes completely through the leaves, and some of the new growth on the Walmart-refugee has been skeletonized completely. What munches like this and is there any way to stop it?

PS: sorry no pictures, RIP my digital camera