is the conditionnel only after the "au cas où"

Yes, the conditional is used in a clause that follows ‘au cas où’ - this is noted but not overly emphasised in the lesson. In the first example, there is a completed action done ’‘just in case’ of something that is not expressed eg I bought some more (umbrellas), in case “it rains” - au cas où il pleuvrait. 

With the second example, expressed as an incomplete sentence, as is common in speech - ‘je vous ai fait un sandwich, au cas où’.