Why do scientists think there is an undiscovered carbon sink somewhere?

The major carbon sources (reservoirs that release more carbon than absorbed) are: deforestation and fossil fuels

The major carbon sinks (reservoirs that absorb more carbon than released) are:
soil, oceans and plants.

When scientists calculate the amount of carbon dioxide is returned to the atmosphere vs the released the amount of carbon, a large amount is unaccounted for and the total does not add up. This is why scientists believe there is an undiscovered carbon sink somewhere.

When scientists calculate how much carbon dioxide is release and how much is returned to atmosphere and oceans, a large amount is unaccounted for.