There is no requirement for the oil cap to be wire locked on the Grob 115E, which is equipped with a modified (inverted) engine oil system to cater for aerobatic manoeuvres. The maintenance organisation considered that the most likely scenario is that the drain cap was not correctly secured after the engine was installed.
If it is not wire-locked, what kind of "correctly secured" is the MO talking about? Using a torque wrench?