Unexpected high loss of alcohol in wash ?

[Homesweethome]

I am puzzled with a situation. I made a sugar wash with 20 kg sugar mixed into about 120 liter water + baker’s yeast and nutrients added. The fermentation went o.k.
After one week of fermenting the density of the wash was well below 1 and the wash was starting to clear up. I waited for about 10 days after that. Then, just out of curiosity, I measured the alcohol content of the wash and it was only 4% while it should have been about 8% with the amount of sugar and water that I had used.

I am fermenting in a stainless steel barrel that was covered with a loose, but quite well fitting lid. The only explanation that I can imagine now is that the missing alcohol must have escaped via the loose fitting lid. But that much ? Did some of you experience such an high alcohol loss via a loose fitting lid ?