@Forgotten_Frank1 I am a bit confused here. When you say "marks" an enemy, do you mean by scanning them with the Tact? or by pinging their footsteps?
If you mean by pinging their footsteps, keep in mind, all that passive does is let you know how long ago an enemy was in x area. It does not let you know who it was, so it would be confusing not knowing who you'd deal 10% more damage too giving you a 1/3 chance to do so.
If you meant by marking them with the Tact, that sounds a bit more reasonable, however, it would make her Tact pretty much above all else if it was to deal 10% more to scanned targets.
I do like the idea of it scanning for a second more or even half a second to track targets for a bit longer.
I believe where the buff is needed is by being able to scan footsteps or clues you find and than seeing where those foot steps/clues lead. or allow them to be more visible from a distance once scanned.
Ya know, be a true hunter