Certainly on a D5 185 there is an aluminium pipe towards the back of the block. This is out of sight and fits the bill for your problem.
It's caught a few out on here as it is a slow , often not obvious leak.
Why not add some fluorescent dye to the header tank and see where it is leaking.
As already mentioned, head gasket and cracked block are virtually unheard of on the D5.
Read both pages of this thread.
https://www.volvoforums.org.uk/showt...t+leaking+pipe