There are several synonyms for the phrase "he kept an eye on his luggage." One alternative could be "he was vigilant over his belongings." Another option might be "he monitored his luggage closely." Additionally, one could say "he was watchful of his bags" or "he maintained surveillance on his luggage." These phrases all convey the same idea of someone being careful and attentive to their personal belongings. It's important to keep an eye on your luggage, especially when traveling, to ensure that it doesn't get lost or stolen. Using synonyms can help spice up your writing and avoid using the same phrase repeatedly.