I suspect that you are wanting to express "We left early just in case"

Nous sommes partis en avance au cas où  (Juste is not required) and also you cannot have two vowels together in the way you have written "juste au".

So unfortunately you have unwittingly erred on two counts.

Hope this helps and explains your query.

Alan (Jim)

To clarify - you most certainly can say 'juste au cas où'. It is not at all wrong, although adding «juste» does not make a significant difference to the meaning, much as in English ' in case' and 'just in case' have near identical meaning. Either with or without «juste» should be accepted here Au cas où + conditional mood (Le Conditionnel) = in case in French%252Fsearch%253Fs%253Djuste%252Bau%252Bcas%252Bou