Always, always, ALWAYS assess your surroundings. Unless there is an IMMEDIATE threat, you have time to figure it out. I have never heard a sonic boom-chances are, if I ever do, I will likely jump out of my skin . But, as others have said, you sort of look for secondary indicators, and assess based on that; smoke, fire, building collapse, care wreck, whatever. For all we know, they could be building a bridge, and blasting rock (I know, they use blast blankets, etc...just trying to illustrate a point). The biggest thing is, remain calm, and take in your surroundings. Assess the situation, and base your course of action on whats going on. To be honest, I wouldve done the same thing you did; nothing. Once I observed that there was no danger, I would assume whatever the noise was, is now someone else's problem.