I actually see them at the surface a LOT, and often in groups. Sometimes but not always, they are "clooping" - tube mouths working the surface for
something, but nothing I can identify.
I've never gotten them to eat any dry fly.
A week or so ago, there were carp, both singular and in groups, just milling around under the surface on a Basin lake. Some had their backs poking out of the water, but only saw one (out of dozens) with lips up.
Casting just outside one of the groups, I did get
one to chase and eat a slow-stripped, glass bead head, black marabou leech just under the surface:
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