I understand that Windows Sandbox 'provides a lightweight desktop environment to safely run applications in isolation and that software installed inside the Windows Sandbox environment remain "sandboxed" and run separately from the host machine' etc, etc, however, surely it must leave a 'footprint' somewhere. How do I find this footprint on a PC running Windows 10?
Any machine made with Windows Sandbox goes away at Restart or Shutdown.
Sandbox is not an Installed Program.
Look in Programs and Features, Windows Features. Scroll down to "W" and look for Windows Sandbox.
If it was used, it will have been enabled and likely will remain enabled.
A user (with Admin Credentials) could have enabled, made a Windows 10 sandbox, deleted it, and removed the setting, but that does not seem completely reasonable.
So look in Windows Features and see if Sandbox has been enabled.