Infiltration

[Hina Muqadas]

The act or process of infiltrating, as if water into
a porous substance, or of a fluid into the cells of an organ or part of
the body.

[Couch Potatoe]

To [sneak] into a site, usually active for the purposes of [urban exploration]. Can be used as either a noun or verb.

Ninjalicious [zine] and website.

[Marquetta]

The action of entering or gaining access to an organization or place surreptitiously, especially in order to acquire secret information or cause damage.

[Luis Fok]

the process of secretly becoming part of a group in order to get information or to influence the way that a group thinks or behaves: