stop project that started without debugging

No. "Start Without Debugging" spawns a new, independent process. Even shutting down Visual Studio won't terminate it.

Unless your project is a web project and uses the ASP.NET Development Server. That one VS actually shuts down on exit.

I stumbled upon this answer after misclicking on "Start without Debugging" and not being able to find my process in Task Manager.

In Visual Studio 2015 I was able to go to Debug->Attach to Process and find my process in the Available Processes list (named after the solution in my case) and attach to it.

My understanding from https://blogs.msdn.microsoft.com/zainnab/2010/11/01/start-debugging-vs-start-without-debugging/ is that this is the same process that "Start Debugging" does after it starts the process.

Then in the Debug menu there is a Terminate All option as described in this other question: What's the difference between "Stop Debugging" and "Terminate All" in Visual Studio?

You can find iisexpress icon in tray, and stop site: