Like Monty Burns, I bided my time. Oscar and Wee Wee continued their escapade under the shearing floor. I knew they were still there because one, Wee Wee I think, would grunt occasionally.
I put their food out twice a day, and twice a day they would sneak out from under the floor and chow down, racing back under the floor at the slightest sound. Such as me peeking around a corner.
So I moved their food dish further and further away from their escape hatch. And then I opened a gate into the adjacent section of the barn (which I totally, totally secured) and I place the next feeding in that area.
In they went and slam went the gate.
I returned to the offending escape hatch, dug a trench down and installed more facing boards, so to closing the opening, both above ground and about 18″ below ground.
Next feeding, back in the intended, and hopefully now secure, home pen!