The core rulebook first abstractly states that circumstances should be illustrated with bonuses or penalties during a opposed test.
Core Rulebook at chapter 9, page 213 regarding Opposed Tests :
you should grant bonuses or impose penalties of 1-3 to reflect
the circumstances of the test
Factors you may consider for bonuses and/or penalties to
ability tests include equipment available, weather conditions,
time constraints, distractions, assistance from others, lighting
conditions, and good roleplaying where appropriate
Those rules, as you can read a little further in the book would also apply on attack rolls, moreover those special circumstances are a bit described.
From the Core Rulebook at chapter 9, page 216 regarding Attack Rolls
& Circumstance :
You can also assign bonuses and
penalties to the attack roll to reflect the circumstances of the
encounter, just as you do in opposed tests. In general these
bonuses and penalties should range from 1 to 3. They can take
into account anything that would affect the attacker’s ability
to hit the target, such as lighting, terrain, tactics, and concealment.
Sneak attack ability already reflects the fact that your avatar deals extra sneak damages on a target he is not already engaged with. If a bonus would be applied it should respond to a particular/unuasual situation, as brought by the player for example.
In the end bonus is always up to the GM, but rules as written should incitate your GM to give a +1 to +3 bonus to reward good or successful tactical approach.
On that same page, there are some examples on the Attack roll modifiers table with bonuses and maluses that apply and in which situations :
+3 : Defender is unaware of the attack.
Given a target unaware of the player, facing the other way. From the above rules, I would allow a player to :
- Sneak on the unaware target maybe with a -1 to -3 penalty (depending on game factors)
- Do a sneak attack with a +3 bonus upon successful Dexterity (Sneak) test against target's Perception (Hearing)
- Apply sneak attack damages upon successful attack roll