Normally terminating the telephone connection (hanging up) would automatically return focus to the previous foreground app without any action necessary. At least it works that way with Maps and Co-Pilot so I assume it would work the same way with Google Maps. However I have occasionally gotten myself into an indeterminate state that is hard to escape — especially when distracted by traffic.
Usually a deep breath to calm my thinking — followed by a single click of the home button — a pause to allow one app or another to come to the foreground — then a double tap of the home button to bring up the open app tiles so I can select the one I want will work. But — it is one thing to do that sitting at my desk or in a parked car, and an enormously different and more dangerous thing on Interstate 20 traveling 75 mph and playing bumper tag with dozens of 18 wheelers desperate to get to the other side of the metroplex before the rush hour traffic build up.
can't Siri get Kevs back to his phone?
Maybe in iOS 11 when it ships, but not in iOS 10.3. Besides that Siri has trouble interpreting my terrified screams when I am in heavy traffic and need to now where to turn
NOW.
If the problem recurs, I would while sitting quietly at my desk and not in the car and in traffic, start by rebooting or resetting the iPhone. If that didn't put a stop to it I would delete and reinstall Google Maps. (I have had to do that with at least one ill-behaved app.)