Find my iPhone - security breach?

Dear Apple community,

Here is a Find My iPhone brain teaser. I have an iPhone and if I track it via Find My iPhone it appears in site A, a work place. That is where it should be. However, there are times when the device appears in a second site, site B, which is approximately 3 to 4 direct miles away from site A. When it is located in site B it is not a wide green circle suggesting its whereabouts. It quite accurately locates the phone inside a building which must be 25,000 square feet and set in its own grounds. The phone will appear there for a while and then after a new Find my iPhone search or a after some minutes the phone sometimes goes back to site A. I've observed this happening generally through the hours of 9am to 3pm and on several occasions. You might also like to know that site A and B are both in a city.

Firstly, the phone has never been in site B, ever neither is the route taken to site A any where near site B. Secondly I am tracing its position from another iPhone, iPad and a MacBook Pro that are located over 15 miles away. They all show the same mysterious location issue. The only common factor between site A and site B is that there is a person, a colleague, who works part time in both sites. My concern is that there is some sort of security breach. Can anyone advise?

Looking forward to your replies.

Mike