Is it permissible to monitor a person around whom suspicions have gathered that he is a thief?

The default rule is that it is not permissible to spy on Muslims, based on the Almighty's saying: "And do not spy," and the Prophet's (peace be upon him) saying: "O community of those who have believed with their tongues but faith has not entered their hearts, do not backbite Muslims, nor seek out their faults. For whoever seeks out their faults, Allah will seek out his faults, and whoever Allah seeks out his faults, He will expose him in his home." However, if there are indications pointing to the existence of prohibited acts, the rectification of which is feared to be missed, such as a trustworthy person reporting that a man is secluded with a woman to commit adultery with her, or a man to kill him, then spying is permissible at that point to avert that. But mere suspicion does not justify surveillance and spying.