The nose knows...
Most people can recall someone in their lives who had a nose for trouble, be they the motherly figure that knew when mischief was brewing, or the fiend that always knew when trouble was afoot. The nose is also linked to various other expressions, all with the implication of some "higher" purpose or greater knowledge. Take for instance the expression: "Being led by the nose", or describing someone as noseying around. It now seems that many of these expressions are being supported by scientific discoveries, proving that there is a very good reason that the nose knows.
In a range of experiments towards understanding the human brain and how it works, scientists have found that our noses play an important role in a wide range of human experiences. Findings that our sense of smell and emotions activate the same areas in our brain begs the question whether there is any link between our emotional well being and where we have been poking our noses?
In addition, our sense of smell seems to be active even when we do not notice any, and it has been proven that what we smell may influence what we visually perceive when the odor is linked to memories with negative emotional content. What we smell may therefore filter or color what we see, and in the case where someone was being led by the nose it may just be that the person they following was pleasant on the nose.
Unfortunately no-one can blame their nose for anything because it also seems that we have a semi-conscious ability to increase our perception of smell which causes the neurons in our olfactory bulb, the area in our brain responsible for our sense of smell, to light up in anticipation even if there is no odor. It also gives us the ability to adapt to unpleasant odors, allowing us to ignore them when they permeate the air we breathe from day to day.
All of which still does not fully explain the fact that my mother could smell out secrets, but there seems to be enough reason to pay attention to the person who is smelling the rat. As far as the expressions go I believe they may all have some origin in fact, even if those facts are only now being discovered by science.