Couple of years ago I bled the radiator on my GT4. Forgot to replace the expansion tank cap and only realised after driving only a couple of hundred meters. Could I find the cap anywhere? No. Bought a new one.

Fast forward a year when we had a CS open day at HR Owen service where I had my car inspected. They pointed out a very frayed AC belt. Took it to QV to get it replaced and they thought the AC compressor was running rough and needed a new bearing. Went back next day and they produced a very mangled expansion tank cap which had been jammed in the AC compressor pulley.

Lesson learned. ALWAYS find the missing bit/tool - and always remember to replace the cap!