What could cause two different iphones to incorrectly show my address as their location?
Pure guess, but …

The geomapping for the local WiFi connections is incorrect.
Somebody recently moved house &amp; took their router with them, or bought one from someone else, etc, etc &amp; the locations database has not been updated to reflect this.  

Find my iPhone relies on already 'knowing' the position of all available WiFi APs [irrespective of whether or not it can or has ever connected to them] to triangulate the current location.

See How Google-and everyone else-gets Wi-Fi location data for a background story.
&amp; HT5467 Updating WiFi router location in Apple's database? for some ramblings on what happens when the database doesn't get updated.

As far as a guess on why two different people had the same issue, that has me stumped, unless the 'owner' of the out-of-position WiFi is also the source of the vanishing phones.