the mind > attention and judgement > enquiry > investigation, inspection > secret observation, spying > procedures used in spying > [noun] > infiltration (5)
penetration1903
Infiltration of a country, organization (esp. a political party or enemy spy network), etc., in order to gain power or information.
infiltration1940
The action or process of infiltrating; percolation. figurative. (Cf. infiltrate, v. 4.)