This is par for the course when you rely on borrowed or 'stolen' art; sure, maybe you fly under the radar, but when you stop flying under the radar, or when you move from a modding community where it's not worth anyone's time to call you on it vs. one where other modders and the dev take an active hand, there isn't any sane defense (you can argue that the laws ought to be different in the abstract, and maybe they should, but the dev's interest1 is in avoiding trouble with the laws as they are now).
The only way to ensure you can do literally whatever you want with an asset is to make sure you... own the rights to the asset. Everything else is some degree of hoping not to get caught or face any consequences.
1Even though we're past the days of individual music pirates being prosecuted, that didn't stop because it stopped being against the law; it stopped because it was impractical and bad PR (and PR, not legality, is the reason IP mods are mostly left alone). But if you run a forum and you routinely allow flagrant and actionable copyright violations... you might be worth going after.