that Col Brandon is "self-conscious about some mysterious woman somewhere" in Chapter 11:

"This," said he [Col Brandon], "cannot hold; but a change, a total change of sentiments -- No, no, do not desire it, -- for when the romantic refinements of a young mind are obliged to give way, how frequently are they succeeded by such opinions as are but too common, and too dangerous! I speak from experience. I once knew a lady who in temper and mind greatly resembled your sister, who thought and judged like her, but who from an enforced change -- from a series of unfortunate circumstances" -- -- Here he stopt suddenly; appeared to think that he had said too much, and by his countenance gave rise to conjectures which might not otherwise have entered Elinor's head. The lady would probably have passed without suspicion, had he not convinced Miss Dashwood that what concerned her ought not to escape his lips. As it was, it required but a slight effort of fancy to connect his emotion with the tender recollection of past regard.

Elinor's observations are not always correct, as nobody's would be. I believe that she may be correct, however, in concluding that Col Brandon thought he had said too much, thought he ought not to have mentioned this lady of unfortunate circumstances.