shadowing
1. [ adjective ] following surreptitiously; keeping under surveillance
Synonyms: tailing
Examples:

"always on guard against shadowing submarines"

Related terms: following
2. [ noun ] the act of following someone secretly
Synonyms: tailing
Related terms: pursuit chase
Similar spelling:   shading
  shedding
  shadowiness
  sheathing
  shadiness
  shoddiness
  shutting
  shouting
  shooting
  shitting
  sheeting