(n.) The practice or employment of spies; the practice of watching the words and conduct of others, to make discoveries, as spies or secret emissaries; secret watching.

bugging cloak-and-dagger work counterespionage counterintelligence electronic surveillance espial following intelligence intelligence work looking lookout military intelligence note notice observance observation reconnaissance regard respect secret police secret service shadowing sleuthing spying stakeout surveillance tailing trailing viewing watch watching wiretap wiretapping witnessing