Can a Project Be "On Track" and Still Losing Its Key Resource Every Day?

A project can look "green" on the dashboard while its most important person is quietly getting pulled into other work.

Example: A developer shows "80% allocated" on paper, but he's the only one who knows a legacy system, so every team pulls him in for "just 10 minutes." Those small pulls add up daily, and the project slowly slips even though status stays green.

In my opinion, utilization % doesn't catch this. Simply asking "What are you being pulled into that's not on your plan?" usually reveals the real bottleneck faster than any report.

How do you catch this kind of hidden resource pull before it causes a delay?