If my memory serves me right, we had a 763 that was losing centre fluid on every sector. When it was topped up on the ground and tested, no leak could be found. There was no evidence on the airframe of where the fluid was leaking from and eventually it went into the hangar for further investigation.
It was jacked up and gear swings carried out. In the end, it was found that the tailskid was leaking, but only when "in the air". Apparently, hydraulic pressure is applied in flight to keep it retracted.
This was a few years ago, but I think the details are correct. Certainly foxed everyone for a while at the time.