Antonyms for the phrase "checked up on" may include "neglected", "ignored", or "disregarded". These words imply a lack of attention or concern, indicating that no follow-up or monitoring occurred. Other antonyms may include "trusted", "relied on", or "believed". These words suggest a high level of trust and confidence in someone or something, indicating that no checking or investigation was necessary. Additionally, "trusted" or "relied on" may suggest that the person being trusted has been thoroughly vetted and can be counted on to perform their duties without constant oversight.