A scope gap is a piece of work that falls between the cracks of the awarded subcontracts — each trade assumed someone else had it, so nobody bought it. Classic examples: connections between steel members, blocking behind drywall, or final cleaning nobody's contract named.
Scope gaps are the reason bid leveling matters so much: they're cheapest to catch during buyout and most expensive to discover in the field, where they show up as change orders and finger-pointing. A big part of a PM's value is thinking in whole systems so the gaps get assigned before they bite.