When the bosses see me, does that mean I won't be able to get the achievement? Thanks

Foxiest of Hounds requires only that you get the Smooth Operator bonus for all missions in which that is possible.

Smooth Operator is gotten by not triggering any alarms where such are present during a mission. There are not that many of those as we are talking specifically about alarms like the ones you see on the walls. Alarms triggered by the game itself as part of gameplay/story do not count.

You can also be seen all you want and still get Foxiest of Hounds.

alright thanks for the help, last achievement to go ^^