Something is eating it - probably some type of caterpillar (larva). Could be quite small. The eaten piece doesn't look fresh so it's probably moved on. Unless you're getting a lot of this type of damage, it doesn't harm the plant.

Maybe a furry creature? My dog likes to nibble on my green bean leaves. Haven't ever noticed her going after the pepper leaves though.

Try elevating the plant.
If that doesn't work, try elevating the plant on blocks over a tray of water.
If that doesn't work, perhaps the pest is in the soil itself.

Josh

Try using a flash light on the underside of the leaves to help you find critters' shadows

Or it is apparently eating the leaves, so any form of residual poison would do the trick. Residuals don't do as well against sap suckers, but anything eating the leaf itself will die a fairly quick death from a quick spritz of sevin.