I wouldn't have expected it to have gone off either, especially with the amount of oil in it

Mikey, are you certain that it was off? The aroma of the base will develop over a few days and not necessarily for the better, IMO.
I have left a big pan of base on the hob for three day's before now (being an idle sod) and it was fine, well, I think it was, I lived to tell the tale anyway ;D
On a similar matter, when I defrost a portion of the Taz base, it looks absolutely minging but, a quick stir and it looks fine again.