Most of the words suggested so far are adjectives; the best of them are vigilant and watchful. 'Proactive' is alright, but not as specific; someone who is proactive is thinking ahead and taking steps now to accomplish their future goals, but that agenda might not include identifying and resolving systemic problems.
For a substantive, I think the word that most matches the OP's request is:
1: a skilled worker employed to locate trouble and make repairs in machinery and technical equipment
2: an expert in resolving diplomatic or political disputes : a mediator of disputes that are at an impasse
3: a person skilled at solving or anticipating problems or difficulties
Other suggestions for substantives include "Scout and Lookout". These occupations look for danger and react, but the dangers they are focused on are external to their organizations, and OP appears to want something more internal.
Similarly, a word that has not been suggested is Watchdog:
An individual or group that monitors the activities of another entity (such as an individual, corporation, non-profit group, or governmental organization) on behalf of the public to ensure that entity does not behave illegally or unethically
but here again it has the connotation that the individual is outside the organization that they are monitoring.
A more internal connotation is carried by Auditor and Inspector. Here, however, the people generally look for breeches of established procedure or protocol. They don't necessarily concern themselves with future problems, issues that have not yet arisen or been codified.
A whistleblower, as already noted, does not intentionally look for problems, but reveals them once found. The problems themselves are deliberately concealed, which does not match the OP's meaning. The same answer suggests 'debugger', but this only carries currency within the tech field, and see also this comment.
The best match for what the OP is describing in the substantive is Troubleshooter.