To my shock... Someone else had a phantom in literally the same area I live (don't know who - just saw it in the sky) - and was flying straight overhead!

Question though... Was wondering - if there are two phantoms in the same area (or, as these become more popular, let's say 5... or 10... etc) - will the signals 'interfere' with each other?
What happens then - does it just return home?

You will not get any more interference from another Phantom in the area than you would normally get. The radio frequency the Phantom uses is shared with every WiFi and a bucket load of other non-essential services, they are far more likely to cause you interference than another Phantom. If the radio spectrum gets to crowded, you will get a warning on your screen and it most likely will stop and hover in place.