Even if you have a passcode set to keep your phone inaccessible to other people, Siri's default setting is to ignore this. By holding the home button to activate Siri, anyone can bypass whatever preventative measures you have in place.
Don't despair—it's a simple fix. Start up Settings and navigate to General/Passcode Lock. Make sure that the “Siri” option is set to “Off.” That's all it takes to disable Siri while a passcode is in place.