One day, John obtains the gift of clairvoyance. Since he is a good guy, he'd like to use his gift to help humanity. And of course there are many good things he could do, like finding where missed or kidnapped people are, or safely obtaining information from dangerous places.
However there's of course also the danger that intelligence agencies around the world might become aware of him and either want to force him to work for them, or to eliminate him in order to prevent him to reveal their secrets.
Here are the properties of his clairvoyance abilities:
- He has to explicitly mentally "visit" a specific place and time to get any information from that place. Especially he will not learn about any event (such as, some intelligence agency planning to kill him) unless he happens to mentally visit that specific event.
- He is a clairvoyant, not a soothsayer: He can only mentally move to places/events currently in his past (more exactly in his past light cone).
- He only can experience whatever he could have experienced had he actually been there; so he can see visible light (but not UV,IR or similar), hear audible sound (no infrasound or ultrasound), smell, feel temperature, etc. However his "mental senses" are capped so that his mental experience will never be painful (but he knows if that happens).
- Besides those perceptions, he has no further abilities; no mind reading, no sensing of magnetic fields, no direct reading of hard disks or network communication, and no automatic understanding of foreign languages (for example, if two people on his visited place speak in Chinese, he won't know what they say since he doesn't understand Chinese — the Chinese intelligence agency planning to kill him might therefore go unknown by him even if he happened to mentally visit the meeting where it decided).
- To mentally visit a place, he mentally moves smoothly through space and time. He obviously can always start wherever he currently is, but he can also "mentally bookmark" the place and time he's currently visiting, and then instantly return to that event by mentally "invoking" that bookmark. However those "bookmarks" otherwise act like normal memories: As they are not used, they tend to become more and more fuzzy (meaning when he invokes them, he might not get exactly at the bookmarked event, but possibly a bit earlier/later and/or at another, nearby place), and ultimately he well likely forget them.
Given those limitations, it's clear that he certainly will not want to risk any intelligence agency (or other organization that has something to hide, like, say, the Mafia) getting aware of him.
But with that limitation, how could he still use his ability to help humanity? One obvious way would be to anonymously call the police on missed/kidnapped people cases, but the knowledge about the case will be seen as indication that the anonymous caller is somehow related to the case, and cause the police to look for him; if he does it too often, chances are high that they ultimately will track the calls back to him.