I haven't had it happen to myself yet but I know a lot of people who suffered from it. From straight out stealing the OC or art to tracing over the artwork and shittily editing the artwork. I also herd enough of people who get their art stolen by companies for mass production, often in china or somewhere where it's hard to contact the company and even get through...I always watermark my stuff(unless I forget it woops) and always ON the art and not...idk...conveniently next to it so it can be easily edited or or something. Better be save than sorry.
EDIT on the topic of tracing...I also firsthand saw someone do that. Like...they fucking screenshoted their process and had the layer they were tracing over visible.