Please help with a short Dementor question.

[ghinghapuss]

No: HPFGUIDX 85213

Help!
Did I read somewhere in OotP that the Dementors had left Azkaban and 
joined Voldemort?  I do remember the mass breakout of Death Eaters in 
Chapter 25 "The Beetle At Bay", but I can't recall if it was because 
the Dementors had left the prison leaving it open for the prisoners 
to escape?

Does anyone remember if the Dementors departure is canon? If so can 
you give me a page quote? Or perhaps it's some theory I read 
somewhere?  I've tried the Lexicon but no luck.

Thanks
Mandy