How can one find out if a spouse has a hidden second phone? Any tip offs? It would not be on our telephone plan as I can see all the numbers on the family plan. I am talking about those throwaway phones or something. Seems that is the way they are going now.

The only way I know is to snoop around and to also put a voice activated recorder in places you think he might use the 2nd phone.

There are several possibilities, depending on just how careless one gets. If he links his appleid with the new phone, it could show up there. Check FaceTime to see if there is suspect history there. Most likely, the new phone would be a burner with pre-purchased service.

I would get ahold of the iPhone you know about, go to Settings:Privacy:Location Services:System Services:Significant Locations. Look for something in there that looks suspicious.