Some IDG's have a traction type CVT that is very sensitive to oil temperature, flow rate of oil for cooling, and very tiny debris particulate contaminants in the oil. The oil filter system typically includes a bypass valve that allows a continued flow of oil in the event the filter element becomes sufficiently clogged to prevent adequate flow of oil for safe operation of the CVT. The filter system often uses a differential pressure sensor (sometimes called an impending bypass sensor) to indicate that the oil filter element requires replacement.
Depending on the particular IDG design, it might be possible for the IDG to operate without having a filter element installed. The oil filter differential pressure sensor and bypass valve are typically designed to deal with a drop in pressure from the filter inlet to the filter outlet. If the absence of a filter element did not cause leakage or disrupt oil flow from the filter system inlet to its outlet, then it might function without any indication of a problem.