>> What infective agent could cause foragers to not return to hives?

And the data shows that it is not just foragers in the case of CCD.

House bees also are leaving never to return, which results in 
greatly reduced numbers over a short period, sometimes far 
fewer than required to support the brood one finds, proving
just how quickly the population was depleted.

While it is plausible that massive forager losses could prompt
immature bees to take up foraging, this would be detectable
through reduced stores of fresh nectar and pollen when the
population is clearly depleted.  So we can conclude that more 
than foragers are leaving, never to return.

The implication here is that foragers (or robbers) are bringing 
one or more pathogens into the hive, as the spread is 
pathogen-like, and the symptoms are very unlike pesticide kills, 
where some bees survive, but show symptoms of pesticide exposure,
such as the classic "shakes" associated with many of the neonicitinoids.

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