Das Gesindel hat sich darin verkrochen. Wo auch sonst!
I wonder if this expression means:
Where else?!
or:
Just as usual!
I'm not sure how to interpret the precise function of auch and sonst in this specific instance.
The expressions of the form wo auch/denn sonst?
, wie auch/denn sonst?
, was (auch)/denn sonst?
imply that the speaker can think of no other possibility, or other possibilities are just not as good as the possibility at hand, or s/he might actually be stating a question, expecting an answer (only if there is actually someone else around to answer).
Wo auch sonst?
Where else would they go?
implying that there's simply nowhere else for them to go or this is by far the best place for them to be or the speaker doesn't know about a better place.
Wie denn sonst?
That's the only way! // However else would you do it?
implying there's no other method of doing something or the applied method is superior to others or this is the only way the speaker knows how to do something.
... is just ...
Where else?
It's a simple word by word translation:
What obviously makes problems to you is the word ...
This word, as it is used here, is a modal particle.
I have written an answer about modal particles in German language: https://german.stackexchange.com/a/12242/1487
But you might also find this article in Wikipedia useful: German modal particle
The most important facts about modal particles:
The translation you gave are fitting: Where else?! or Just as usual.
It could also imply that the speaker is kinda angry, or that it's quite obvious, that the 'Das Gesindel hat sich darin verkrochen'
A literal translation of "Wo auch sonst?" is "Where else otherwise?"
A figurative English translation is "what else can you expect?"
The expression is better translated as "of course"
"Das Gesindel hat sich darin verkrochen. Wo auch sonst!" -> "The rabble have gone into hiding there, of course", or "of course the mob have hidden themselves in there", or even "The riff-raff have crawled in there. Of course!"
Generally "Wo auch/denn sonst?" implies heavy sarcasm, rather than a literal interpretation: "Wo ist dein Mann?" "Er sitzt in der Kneipe, wo denn sonst?"