Rights and Duties Management System (RDMS) will be one of the very powerful customization systems in DU. Read the DevBlog here if you have not already. Effectively tags can be used to give access to assets etc etc. I assume that in the current RDMS system, a player will know what tags he/she has. Effectively these are like keys to a spaceship or passcard for a door.
The suggestion of this topic is that a tag can be placed on a person without their knowledge. These invisible tags would not be able have duties connected to them because this could lead to a player losing money without knowing about it.
The uses for this are also quite broad. For example: it could be used to not only tag intruders but to keep track of the intruder long after they have left. This has an additional effect that invaders would need a hacker to look at their hidden tags and clean out any unwanted ones.
What would you use invisible tags for?