The word "lurk" doesn't exactly exist in French. "se rôder" fits in some cases, but another translation "se cacher" (to hide) fits others. I'd write "X crept into Y" as "X s'est glissé dans Y", but that has a connatation moreso as an accidental short-term mistake. I don't know how I'd express the idea concisely in French. Also hard to tell exactly how he wanted to convey it, something between "crept", "were hidden", "were lurking" probably? As I've discovered the hard way, there is not always an analogous term for the same fundamental idea/concept between two given languages; mastering the nuances of these differences is important for proper fluency. I probably make errors like this frenquently writing/speaking French.
Edit: They appear to be french: http://serge.liyun.free.fr/serge/