What diagnosis to you suspect?
Subdural empyema - gas and peripheral contrast enhancement are worrying signs.
The surgical bed and extraaxial collection demonstrates peripheral enhancement, extending into sulci. Two gas locules are demonstrated which were not evident on on the previous CT.
Conclusion:
Although a least a proportion of the enhancement demonstrated could be post-operative, increasing size and density of the fluid collection, and enlargement of the extra-axial collection and development of new gas locules is suspicious for an empyema.