Conscience: the inner voice which warns us that someone may be looking.

H. L. Mencken (1880-1956) American writer and journalist [Henry Lewis Mencken]

A Little Book in C Major, ch. 4, § 12 (1916)
    (Source)

Variants:

CONSCIENCE. The inner voice which warns us that someone is looking.
[A Book of Burlesques, "The Jazz Webster" (1924)]

Conscience is the inner voice which warns us that someone may be looking.
[Chrestomathy, ch. 30 "Sententiae" (1949)]