Sometimes during ground combat, I see orange spinning target things on NPCs. Sometimes they appear on enemies, but more commonly they are on securtiy escorts that I or other players beam down. I thought it would become clear eventually, but no. These symbols must mean something - right?

Those are probably the "Expose" markers. It means that the target is "Exposed" to concentrated fire, and the secondary fire of certain weapons (compression pistols, sniper rifles, etc.) and certain melee attacks will deal extra "Exploit" damage. If an energy weapon Exploit attack hits and kills an Exposed enemy, they'll disintegrate.

What you're seeing are called "Expose Marks." A target that has been Exposed is vulnerable to attacks classified as "Exploit Attacks." If you use an Exploit attack against an Exposed target, you will deal around 300% damage on that attack, and if it kills them it has a very good chance of vaporizing them. This mechanic is why it's a good idea to have both an Expose weapon (like a Minigun, or Wide-Beam pistol) and an Exploit weapon (like a Sniper Rifle or Compression Bolt Pistol) both equipped on your Captain.

Your Security Escorts also come equipped with an ability called "Draw Fire" which places a fake Expose Mark on them for something around 30-60sec. This fake Expose is intended to trick other players into firing on the targets using Draw Fire in PvP scenarios.

*edit: Ninja'd by a dev

=)

do PvE NPC enemies target your exposed security escorts? *never paid attention to that*

It increases threat generation, so in theory they should.

In PvP it's not useful on your NPC allies, since people are probably going to have their settings set so that they only target players, but if you have the power active on yourself it can make players waste their exploit shot (unless they saw the "Activating Draw Fire" your character said).

also if its a true exposed person / npc the secondary shot and the melee strike will flash with a x over it saying its a real exposed hit, if it isn't a real expose "Draw Fire" it wont flash

and if your exposed, its best to use a Large Shield Charge because it will "Removes expose effects for 3 sec"

here a vid explaining the flashing (well not really explaing but its there, 3rd item in tray) and showing how to land a exposed shot "Not Mine, Just Found it online, I take NO credit for this video"

LOL found another one, this is more clear, and has some guy talking
Star Trek Online Expose And Exploit Trailer