If the seal header tank oil levels are rising, then the lip seals either side of the Sterntube volume are leaking. This should be monitored, and plans made for repair. If the seal header tank oil levels are not rising, then either there is a system pipework leak (trace and rectify) or oil is leaking outboard of the vessel. This should be investigated immediately with a visual check at the eft end of the vessel and a dive inspection if required.