Well, there still is a voltage there, it's just not referenced to anything. You can still read it, it might just not be a useful reading.
It's completely possible that it's referenced to ground through some other connection. Some signal generators have ground-referenced outputs just like the scopes have ground-referenced inputs.
Now, that means that the ground current has to travel all the way up the instrument's power cable, to the wall outlet and down the other instrument's power cable - meaning it's not a solid ground connection, and at higher frequencies it won't work. That's why you need a close, low-inductance ground connection directly at the probe tip for accurate probing.