Addendum to your point zero—test against a known live wire before _and_ after if you want to be really cautious. It’s possible the tester failed after your initial test, but before you checked the wire you actually wanted to test.
For example, imagine some batteries on the verge of death. They may keep enough charge initially to test right after being turned on but quickly die before you get to the actual wire under test.
For example, imagine some batteries on the verge of death. They may keep enough charge initially to test right after being turned on but quickly die before you get to the actual wire under test.