Foxiest of the Hounds

Foxiest of the Hounds is an achievement/trophy in Deus Ex: Human Revolution. It is earned by completing the game without setting off any alarms.

The easiest way to track this achievement is to check whether you get the Smooth Operator bonus for completing a specific main mission objective, list of which you can find below.

It should be noted that simply getting all the bonuses does not guarantee the acquisition of the achievement. If, for example, the player leaves a body of a knocked out enemy in a place where other patrolling guards may potentially find him and completes the objective fast enough he will still get the Smooth Operator bonus. But eventually the body will be found and guards will sound an alarm (it may even happen out of player's sight), which in turn will nullify the Foxiest of the Hounds. So it is recommended to play cautiously and not to trust the XP rewards too much.

For the general strategies on playing stealthily consult the guidelines listed on the Smooth Operator page.

List of objectives in Deus Ex: Human Revolution that award Smooth Operator bonus
There are 21 Smooth Operator bonuses available in the game (22 with bonus mission from Explosive Mission Pack DLC).

Note that "Investigating the Suicide Terrorist" objective has two mutually exclusive ways of resolving it, so it's possible to earn either Silver Tongue bonus or Ghost+Smooth Operator bonuses in Police Station, but not both. If player chooses to talk his way into the morgue this WILL NOT COUNT AGAINST the Foxiest of the Hounds achievement, meaning that player will need to earn only 20 (21 with DLC) Smooth Operator bonuses out of possible 21 (22) for the achievement.

Detroit Police Station

 * M5 - Investigating the Suicide Terrorist
 * 1) Retrieve the dead terrorist's Neural Hub. As noted above this bonus can't be earned together with Silver Tongue bonus.

Highland Park

 * M10 - Following the clues in Highland Park
 * 2) Infiltrate the Mercenary Compound
 * 3) Locate the Mercenary Leader

Tai Yong Medical

 * M1 - Searching for Proof
 * 4) Enter TYM's tower by passing through the Pangu
 * 5) Find and view Van Bruggen's recording
 * M2 - Entering the Dragon's Lair
 * 6) Confront Zhao in the Penthouse
 * 7) Escape and get to the hangar
 * 8) Open the hangar bay doors

Picus

 * M1 - Confronting Eliza Cassan
 * 9) Escape the ambush by reaching a lower floor
 * 10) Get to the funicular
 * 11) Summon the funicular
 * 12) Get inside room 802-11

Detroit - second visit

 * M2 - Finding Isaias Sandoval
 * 13) Obtain Sandoval's location from Taggart. Note that, unlike the Police Station objective, this bonus can be earned together with or separately from Silver Tongue bonus for convincing Taggart.
 * 14) Find and confront Sandoval

Hengsha - second visit

 * M3 - You scratch my back, I'll scratch yours (DLC)
 * 15) Rescue Tong's son
 * M4 - Stowing Away
 * 16) Retrieve Tong's Package from the Shed
 * 17) Place the C4 on Administrator Wang's Desk

Omega Ranch

 * M1 - Rescuing Megan and Her team
 * 18) Search for Signs of Megan's Team
 * 19) Disable the Signal Jammer
 * 20) Upload a virus to the security computer and enter the restricted area

Panchaea

 * M1 - Shutting Down Darrow's Signal
 * 21) Get inside the tower and disengage the lockdown
 * 22) Reach the Broadcast Center

Trivia
This achievement/trophy is a reference to the Metal Gear Solid game series where an identical achievement is also present in the latest titles. The main character, Solid Snake, was a member of a black ops group, "High-Tech Special Forces Unit FOXHOUND". Also, the visual representation for the achievement/trophy is a masked guard with exclamation points above his head, a common sight in all Metal Gear games.