"The charges against the accused are as follows: That he did knowingly, deliberately and in full awareness of the illegality of his actions, having received a previous written warning from the Ministry of Magic on a similar charge, produce a Patronus Charm in a Muggle-inhabited area, in the presence of a Muggle, on the second of August at twenty-three minutes past nine, which constitutes an offence under Paragraph C of the Decree for the Reasonable Restriction of Underage Sorcery, 1875, and also under Section 13 of the International Confederation of Warlocks' Statute of Secrecy."
-- charges against Harry Potter at his hearing, August 12, 1995 (OP8)
Magical activity which might be noticed by Muggles can be an offense under this law.
Harry is warned by the Ministry when they suspect him of casting a Hover Charm in the Dursley's house. The spell was actually cast by Dobby, the house-elf (CS2). After Harry's use of the Patronus Charm against Dementors in Little Whinging, he received a second warning from the Ministry citing this law (OP2, OP8).