System Reference Document

SURPRISE 

If one or more combatants has the drop on the opposition – such as by using Stealth to strike from concealment or set up an ambush, or if combat began with one side holding the other at gunpoint – that character or group of characters gets an entire free Round worth of Actions (one Turn per character with the element of surprise) before Initiative is rolled. 

This is the exception to the rule of "rolling Initiative is the very first thing you do in combat". The GM is the final arbiter of which side, if any, has gotten the drop on the other.