It looks like it'll be a nice full feed too. Sounds like you mean rotten eggshells or the equivalent broken down shell smell based off the description I get that as from what I hear in general when they break down they reek. Personally any eggshells allowed to break down are in the compost bin outside, so I've never let them get there. In my experience though I can forget about a carton somewhere for a month and it's still fine. No bad smell at all.
Or you mean the butyric acid produced by fermentation?