For me, it depends on who is creeping. A junior engineer, I'd probably pull them up and start explaining what is going on. A fellow senior engineer I'd probably take a welcome break and ask them to grab a coffee - the only reason they'd be lurking lol. If it is a PM or someone else it might put me on edge - likely because they wouldn't have the context to understand what I was working on and if they didn't say anything I'd just be wondering what they wanted...