The longer the body was there, the more probable it’d be - as fluids would drain into the soil and decomposing gasses would seep into the soil.
Providing the body isn’t in the worlds biggest freezer zip-lock…
I don’t know that they’d be able to find a 30 year old vertebra buried for 30 years 12 inches deep though; I’d imagine that the kicker is that the ground is disturbed if you just go and bury an old vertebra, while if it’s been there for 30 years, it may be effectively sealed off, or permeated with soil compounds or whatever that might make it hard to smell for a dog.
Trace material of biological origin can persist for a surprisingly long time. We recently watched a documentary on the finding of Richard III’s grave, in the course of which archaeologists came across a late-Victorian-era “thunder box” – apparently a sort of self-contained toilet – the stink of which was still evident even to human noses over a century later.
That a dog could sniff out human remains after a few decades doesn’t surprise me a bit.