I’ve got an 05 Fiesta ST150, mileage of 116k.

Had to remove the sump about 8 weeks ago so it had new oil then. Found yesterday it was totally empty.

There are no leaks (absolutely certain) and it doesn’t appear to be smoking at all, and it’s driving well. I’m worried this might be an engine rebuild job, but would like to know if anyone has any suggestions for what could be causing this before I have to go down the drastic action route?

thanks in advance 

First your need to investigate where that oil went 5.x litres is a lot to go missing.  Great tip if you don't might spending a bit extra, grab some UV dye and Goggles.  Poor some UV dye in the new oil then check for leaks.

Pressure test for head gasket failure too,  iirc the oil filter is on the bottom of the block...is it still there?

You say totally empty, was the engine knocking . There's only two places the oil can go and that's either a leak, and it would have to be a really bad leak to lose that much, or burning and as you say it's neither then the only other explanation is that the oil wasn't there in the first place. I should just top it up and keep a close eye on it.

Hi both, really appreciate the responses, thank you.

Firstly, yes the oil filter is still there! And yes it was totally dry but no knocking from the engine. 
 

Going to monitor over the next few days and then look at options. You’ll hate me for this but I forgot to mention the P0420 code which has been popping up from time to time. That’ll indicate a burning problem as the cats not old. 
 

thanks both