At least within the area of the UK which I lived in until recently the idea that there are lots of unadvertised jobs is a myth. When I was unemployed I did the experiment of applying both via "the usual channels" and also speculatively based upon fairly elaborate analysis to try to identify suitable organisations. The number of "conversions" - which means inquiries turning into job interviews - was vastly higher for…
Plenty of unadvertised jobs will go through networks, word-of-mouth and personal recommendation - obviously cold-calling wouldn't give you good results in such cases.
Generally it's estimated that roughly 40% [for 2009-2010] of jobs are unadvertised, and that's calculated using US BLS data (http://www.bls.gov/news.release/jolts.htm) comparing number of job openings companies state they have versus actual hiring numbers.
While there might be variation between countries, I doubt that there's significant difference between the UK and the US.