Thanks everyone for your replies to my "unknown BOOM" situation. I am glad to know that fewer Rambo's exist (or at least post) then I sometimes imagine. A summary of what I got out of the responses was:

No need to take any large action until you have enough information to know what action is appropriate. However being more alert and actively seeking more information would help you determine the correct action.

Most people were in agreement that of the reaction options I provided, the following was the best, given the limited information available.

11. alertly evaluate the situation with all your senses and with no more information, immediately pay your bill and leave

However there was some discussion on leaving immediately or staying until time or more information determined it was safe to leave.

Other information I thought was useful:

Additional indicators of problems -
Does the building you are in still have power?
Does your cell phone still have signal?
Are people outside your immediate surroundings acting strangely?

Additional sources of information -
TV's that exist in many businesses.
Radio's that play background music in many businesses.
Cell phone (tweets, web, text messages, etc.).
Someone who just came INTO the building.

Things to do -
Go into a heightened (but not necessarily hyper) state of alertness.
Watch the actions of people inside AND outside the building.
Re-check where all visible exits are. Not just the closest.
Put cash on the table to cover your meal. You can then leave quickly if necessary. You can always pick it back up and pay via a different method if nothing further develops.

Miscellaneous -
Note the time. You will then KNOW, instead of GUESSING, how long it has been since whatever caught your attention occurred.
Drawing a weapon will likely make YOU the bad guy in a situation where nobody is sure what is going on.
Don't rush to leave. If the building you are in is unaffected, then you may be safer inside the building than outside.

Finally -
If you take precipitate action (throwing yourself on the floor, fleeing the building, drawing a weapon), this will likely cause reactions in those around you as well. You may be thinking clearly, but if your actions cause others to panic, you may be responsible for changing a minor incident into a situation that gets you or others severely injured or even killed.

Thanks again everyone, I am working on becoming a better survivor and situational awareness is one of my weaknesses. This kind of discussion helps me determine boundaries on what is too little and what might be too much.