TheSagaContinues

Generally speaking, it does seem that hand gestures and the Force tend to go together (though as has been mentioned, there are notable examples where this is not the case).  I agree with StevenO that we as media viewers tend to get those hand motions so we have that ah-ha moment of seeing the Force in action.

I can cite a few things where hiding your Force powers or gestures gives a penalty or another check.

The Force Unleashed book says on page 85 under "Noticing Force-users" that characters who use the Force (and presumably lightsabers as well, given the Jedi association) have unfavorable circumstances (typically -2) to Stealth checks until they escape the public eye (this represents common citizens awareness of Jedi and their general reaction to Force use).  However, the GM can rule that if the use of the Force cannot be visibly connected to a character, this penalty can be negated.

Page 136 and 137 of Force Unleashed mention under "Suspicion of the Force" that Force powers whose activation and effects (such as Farseeing and Mind Trick, specifically, but presumably others with GM discretion) are subtle enough to avoid casual notice (farseeing could be mistaken for daydreaming and the use of Mind Trick might be passed off as the target deciding to be lazy or simply giving into a demand). 

The following powers are deliberately called out as not easily concealed:  force disarm, force grip, force lightning, force slam, force thrust, move object, negate energy, and most applications of surge.  One could presume the obvious reasons for being unable to conceal Force lightning (being a human electric grid seems to be a bit out there) or surge (moving way faster or higher than should be reasonably possible seems fishy).  With the others, the implication is they need gestures, as presented to us by the media.

Now, as StevenO suggests, you can decide how much gesturing is actually needed.  As long as you and your players agree and have fun, that is the most important aspect.  It's nice to know your player is willing to work with you and not against you.  I would personally say for a penalty for Force use, -2 might be better than -5 (YMMV).  And don't forget that NPC Perception checks can be quite problematic for players who want to remain annonymous.

For those that want something more concrete, the Keetael Adept Force talent tree from the KOTOR book has the following talent:

Conceal Force Use
You have learned to use The Force without telltale gestures, reducing the disturbances created in the process. Whenever you make a Use the Force check, as a Swift Action you can make a Deception check to convey deceptive appearances in order to conceal the effects of your Use the Force use.
 Normal modifiers for the Deception's Complexity still apply.

So while not a penalty to the Force roll itself, this talent allows someone to hide their gestures behind a Deception check. which may or may not be successful.  You could fluff it any number of ways.  Perhaps with Move Object, a box moved and hit someone just as I brushed my hair behind my shoulder.  Maybe that chair I threw up in the air was actually my Intercept power so i didn't get shot.

However you do it, remember to have fun and fair.  When I don't want my players to have to track down additional talents, I tend to use those concrete examples as my guideposts for how difficult something should be.  So maybe allow a player to use a move action to conceal Force use without a talent.  Then someone who takes the Conceal Force Use talent feels good about have a better action economy than someone who doesn't.