Yesterday, I was idly perusing the shelves looking for the paella rice when I noticed a small bottle filled with dark liquid.
Closer examination showed that it was indeed the Indonesian sweet soy that I'd previously been looking for. It is, however, quite possible that I didn't see it on the previous occasion because it's not so labelled: the large lettering on the front says 'Ketjap Manis'. No wonder my mental filter didn't see it: it didn't see the words 'soy sauce' on it.