Special thanks to Rare Spawn Overlay's patrol paths and Routes (http://www.wowinterface.com/downloads/info11401-Routes.html)' minimap drawing code for inspiration!
Note: _NPCScan.Overlay can integrate with _NPCScan if you have installed it, but make sure to keep both up to date if you do use both!
Details
World Map
In addition to drawing paths on the World Map, _NPCScan.Overlay adds a key to a corner of the window with each rare's name. It only appears in zones that have rare maps, like Netherstorm as seen in the screenshot. If you need to read something under the key, just move your mouse over top of it and the key will jump to a different corner.
Most map mods like Atlas (http://www.wowinterface.com/downloads/info3896-Atlas.html) will inherit the key and overlays from this module. Even Carbonite (http://www.wowinterface.com/downloads/info12965-Carbonite.html)'s map display will show them.
Battlefield Minimap
The small Battlefield Minimap (AKA the Zone Map) also shows mob paths.
Minimap
All shapes and sizes of minimaps are supported. Rotating minimaps work fine as well.
Also shown in the minimap's screenshot is the found range ring highlighting where Dirkee probably was when the alert went off. That is, Dirkee was seen at the middle of that ring, and he could have been anywhere within the 100 yard ring around that point.
One feature not shown in the screenshot is a thinner 100 yard range ring around your character. It helps you gauge _NPCScan's detection range, so you can sweep through zones more effectively. This option can be found in the Minimap subsection of the options panel, detailed below.
Options
_NPCScan.Overlay's options can be found under _NPCScan's main options menu, under _NPCScan > Overlay, or as its own entry of _NPCScan.Overlay if _NPCScan isn't installed. This panel can be opened with the /npcscanoverlay slash command, or /overlay for short.
The Always show all paths checkbox at the top of the pane overrides the instructions of NPC tracking mods like _NPCScan to hide unused paths, and always displays all known paths instead.
The rest of the configuration pane lists each map display's options. You can enable/disable each map from here, as well as configure individual settings for them.
Each map display can have its overall transparency set independently with the Alpha slider, so that it doesn't obscure the maps themselves.