If it's not specified, that's mean you need to return the loot even if it is an abandoned region.
When you go into decline/end of the game, you not collect the loot in the region you've abandoned (you discard it).
Ok... so if YOU don't collect the loot tokens as they're no longer YOUR region, when an opponent conquers an abandoned region with a loot token in, I would presume they also DON'T collect it as it's not conquering YOUR region? Which wouldn't make sense because then no one can ever collect them after abandoning the region, which makes the rule about leaving them when you abandon the region irrelevant too.
So I'm assuming:
- An opponent conquering an abandoned region DOES take the loot token, even though they're not conquering YOUR region as per the rules; and
- When you go into decline, you DON'T collect loot tokens for abandoned regions, even though the rules say nothing to cover this eventuality.
That seems like the most logical thing (and means the rules for Skags are badly written).