εἰ δέ τίς κα ἐφάπτηται Ἀριστοκράτεος ἐπὶ καταδουλισμῶι, βέβαιον παρεχόντων τῶι θεῶι τὰν ὠνὰν ὅ τε ἀποδόμενος Πατ[ρέ]ας καὶ ὁ βεβαιωτὴρ Μέντωρ. εἰ δὲ μὴ παρέχοιεν βέβαιον τὰν ὠνὰν τῶι θεῶι, πράκτιμοι ἐόντων κατὰ τὸν νόμον, ὁμοίως δὲ [καὶ] οἱ παρατυγχάνοντες κύριοι ἐόντων συλέοντες ὡς ἐλεύθερον ὄντα ἀζάμιοι ἐόντες καὶ ἀνυπόδικοι πάσας δίκα[ς κ]αὶ ζαμίας.
FD 3.3.4 c. 162 BCE
And if someone does capture Aristokrates and reduce him to slavery, Patreas and the witness Mentor assure that this, the sale to god, is valid. And if they do not assure that the sale to the god is valid, they are liable according to the law, and similarly those who are present are authorizied to rescue him so that he may be free, they themselves being impunious and are not liable to punishment or prosecution.
The authorization of strangers to correct an injustice is not unusual for Greek law. For the most part, Greek cities did not have a police force. Neither did they have independent prosecutors. Instead, Greek law was entirely enforced by the private population (primarily citizens, but there are some interesting cases in which metics and other non-citizens were necessary for the enforcement of the law).