How was an I message tracked?

I've got a add on contract to my sisters Iphone. Sent an IMessage to my girlfriend 3 weeks ago. I didn't think they were traceable but my sister plugged her iphone into the computer and somehow found my message to my girlfriend on there and could read it as well? It was only one message she could see though… We send iMessages all day. How on earth was it traced?!

Only one? You sent it to the wrong person or honestly put it in a group text message not paying attention. It would send her all or none, not just one. Best luck.