Green’s Dictionary of Slang

ferret n.3

[SE ferret (out), to search]

1. (US) a detective.

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[US]Ade Forty Modern Fables 64: So the Main Detective called in a couple of Ferrets, who drew Twelve a Week, and they began to Shadow the Young Man.
[US]M.G. Hayden ‘Terms Of Disparagement’ in DN IV:iii 196: ferret, a keen and sly individual. ‘They call detective No. 7 the ferret’.

2. (UK Und.) an informer.

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[UK]V. Davis Phenomena in Crime 161: The ‘ferrets’, the stool pigeons and informers, nose among the warrens of gangland.

In phrases

give (someone) the ferret-eye (v.)

(Aus.) to look at askance, disapprovingly.

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[Aus]J. Byrell Lairs, Urgers & Coat-Tuggers 312: [S]ome of the old Canterbury members and their wives [were] giving my old china the old fidgety ferret-eye [...] staring at him like something left on the loungeroom sheep-skin rug by an incontinent lap-dog.