GUEST, Digger - sorry for not replying sooner but I was in Stony Stratford at the Mudcat gathering.
I know nothing about the machines used for detecting, so I know nothing about "descriminators". No animals would have been harmed by us putting nails down - there was an industrial estate being built on the site. Most of the digs I have worked on are rescue ones, done in the course of construction. Besides, we use nails anyway to mark grids/features etc.
As Naemanson, mooman, Mr Red and others have pointed out, context is the key. But then, since you were an archaeologist for 12 years, you'll know that already. What are your own views on archaeologists vs. detectorists?