Can/Could is type of auxiliary verb that is used to indicate modality – that is, likelihood, ability, permission, and obligation. Now in 'I can smell fire', the author really is using an extra 'can' to express what he smells. 'I can smell fire' is same as 'I smell fire'. See the difference?? It is just an extra/dummy word that can be omitted. Hence, 'empty use'. Same as the other two sentences. You can omit the 'can/could' and it still make sense and now real clear.

Can/Could is a modal, a type of auxiliary verb that is used to indicate modality – that is, to indicate likelihood, ability, permission, or obligation.

But in 'I can smell burning', the author is really merely expressing what he smells, not his ability to do so (in the more usual usage). 'I can smell burning' is same as 'I smell burning' (although the former is by far the more usual way of expressing this, and sounds more natural). See the difference in this usage? Here, 'can' is just an extra/dummy word that may be omitted without changing the meaning. Hence, the term 'empty use' (though John Lawler's terminology is better). Similarly with the other two sentences. You can omit the 'can/could' and they still make sense.