Do we salute officers while passing from behind them?

Yes. The formal & traditional way is to render the salute as you pass and say, "by your leave sir or ma'am".

Yes - FM 7-21.13

4-13. All soldiers in uniform are required to salute when they meet and recognize persons entitled (by grade) to a salute except when it is inappropriate or impractical (in public conveyances such as planes and buses, in public places such as inside theaters, or when driving a vehicle).

As mentioned, if you recognize that a salute needs to be exchanged, exchange it. Simple as that. If you don't recognize the situation, then don't.

I think the FM's language on the salute bears repeating..

4-7. The salute is not simply an honor exchanged. It is a privileged gesture of respect and trust among soldiers. Remember the salute is not only prescribed by regulation but is also recognition of each other’s commitment, abilities, and professionalism.
4-8. Some historians believe the hand salute began in late Roman times when assassinations were common. A citizen who wanted to see a public official had to approach with his right hand raised to show that he did not hold a weapon. Knights in armor raised visors with the right hand when meeting a comrade. This practice gradually became a way of showing respect and, in early American history, sometimes involved removing the hat. By 1820, the motion was modified to touching the hat, and since then it has become the hand salute used today. You salute to show respect toward an officer, flag, or our country.
4-9. The salute is widely misunderstood outside the military. Some consider it to be a gesture of servility since the junior extends a salute to the senior, but we know that it is quite the opposite. The salute is an expression that recognizes each other as a member of the profession of arms; that they have made a personal commitment of self-sacrifice to preserve our way of life. The fact that the junior extends the greeting first is merely a point of etiquette—a salute extended or returned makes the same statement.

I think a lot of individuals forget 4-9 -- it is NOT a gesture of servility of the junior to the senior. For some reason there are many inside the military that think of it this way. They will go out of their way to claim "Sorry, I didn't see you there" or quicken their pace so they can get 'out of the way'. These are the same individuals that will run for the building or their car as the clock approaches 5pm on a military base. The salute is a sign of MUTUAL RESPECT and a tradition between professionals.

I liked the example that was given about saluting...

1LT Thompson and his platoon’s newest NCO, SGT Jemison, were walking toward the orderly room one morning. As they turned the corner and approached the building, PFC Robertson walked out carrying a large box.
PFC Robertson said, “Good morning, sir,” and kept walking past the two. As his hands were occupied, he didn’t salute. However, 1LT Thompson saluted and replied with the unit motto, “First Tank!”

After the soldier had passed, SGT Jemison asked the lieutenant why he saluted since the soldier did not.

“He did by rendering the greeting of the day. If I had been carrying something and he wasn’t, he would have saluted. It’s a privilege, not a chore,” said 1LT Thompson. “It’s just as important for me to return a salute as for a soldier to render it.”

If you can see they are an officer as you pass them, yes. But if you aren't looking at them, how would you know? I don't look at people when I walk up from behind them. Unless I recognize their last name on their PC...how would I know? And more often than not I see officers turn away or pretend they don't see someone because they hate saluting more than anyone.

Ultimately, if you recognize someone is an officer, salute them.