I've had the experience of finding a smartphone and having a hard time figuring out who the owner is. It always seems urgent, since I know the owner is desperately looking for it. Should you find a lost iPhone or iPad, you can simply ask Siri who the owner is. Siri will tell you the owner and show contact information. But you need to phrase the question the right way.
To find out the owner of a lost iPhone or iPad, simply ask Siri: 1) Who owns this iPhone (or iPad)? or 2) Who does this iPhone (or iPad) belong to?
If you ask the wrong way, Siri will tell you to ask Apple or will slip into flippant mode and simply say, "Interesting question." I found that neither of these questions works consistently: 1) Who's iPhone (or iPad) is this? and 2) Who's tablet it this?
Note that this feature of Siri works even if the Passcode feature is enabled.
