On Friday mechanic installed new spark plug tube seals & valve cover gasket set on 96 Toyota Corolla DX 1.8 L engine with 161,000 miles to fix problem of oil getting past the tube seals. I got home & found the semi-circular plug sitting on top of the battery. Based on price of parts ($59), number (JVS-10344), & name of part (Valve Cover Gasket), I would guess mechanic bought a valve cover gasket set without a new semi-circular plug. Can I see if a semi-circular plug is installed on the vehicle without removing the valve cover? Or does shop have to remove valve cover gasket to check if the semi-circular plug is installed? Do any of the following new parts need to be replaced if the valve cover is opened back up? Valve cover gasket, spark plug tube seals, grommets? What happens if the semi-circular plug is not replaced? Is it ok to drive the vehicle back to the shop or will I end up with oil all over the belts, etc? Thanks!