Why do scientists think 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.

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.