Thanks Bel, that clears it up for me. Except what's a KWS

A something something scanner?
Kill Warrant Scanner. Necessary to gain bounties associated with a ship but not the system you are in. Does not give you the right to assault someone who is not wanted in the system you are in. Also necessary to gain bounties in Anarchy systems since those do not have bounties.
As an example: I am wanted in Alliance space and you want that bounty. You need to scan me with a KWS in order to gain this bounty since we are currently in Federation space. I am not wanted in Federation space, so you attacking me will give you a fine and the System Authority Vessels will move in to help me, but if you manage to kill me you gain the bounty and can cash it in in an Alliance system.
Example B is that I have a bounty in both Federation and Alliance space. This time the SAV will not help me, but will instead help you if I fight back. A KWS will allow you to gain the bounty in Alliance space, but the one in Federation space is revelead to you by the normal scanner.
Do note that you actually have to scan for 10s in order to gain the extra bounty, just having the equipment doesn't mean your normal scanner is improved.
What Perseus described is, as far as I can tell, a bug. Apparently most common when other CMDRs scan you when you have stolen cargo.