Is it possible to sneak up on a Force user?

Question

In Return of the Jedi, Luke and Darth Vader both sense each other's presence on the moon. In The Force Awakens, Kylo Ren

senses Han Solo on the Starkiller Base

Is it possible to sneak up on a powerful Force user? Or do you have to be

related to them

or significant to them in some way to be detected? Are there any examples of a Force user not sensing someone (in canon), or any successful attempt at disguising someone from a Force user?

To clarify, I am talking about physical presence, not Force ability.

Han Solo doesn't have the Force, but Kylo sensed him anyways

Answer 1

It's not only possible to sneak up on a force sensitive -- it's one of the few ways to kill them.

Throughout the movies, we see users generally sensing people who are significant to them in some way, be it blood relation, part of their goal, etc.

Vader senses Luke easily enough, but rarely makes mention of anyone else in the group.

In the EU, force users could sneak up on each other via suppression (Force Suppression, mentioned in a previous answer), and generally they only knew when someone who wasn't force sensitive was going to get the drop on them for the same reason they can deflect blaster fire faster than they can see it -- the Force detects danger and alerts them.

In this video, you will find HK-47, an assassin droid, explaining how to kill a Jedi. Aside from mines, gas, explosives, and manipulation the go-to method would appear to be a sniper shot to the back of the head from a long distance. This being the droid that described love as a shot to the knees from 120km away, that distance should be counted as "extremely long" by today's standards -- a tribute to just how hard it is to sneak up on a jedi.

Answer 2

The Huntress (later named Darth Cognus for her instinctive ability to perceive and influence the future), a force-sensitive female Iktotchi, can sneak up on a force user because she has the ability of force suppression.

The dark side also flowed strongly through the Huntress, and with it she was able to hamper the powers and connections of Force-users within close proximity to her. She made use of her talents in a career as an assassin in the years that followed the end of the New Sith Wars.

_{Source: http://starwars.wikia.com/wiki/Darth_Cognus}

There is an example in the novel Dynasty of Evil: Star Wars (Darth Bane): A Novel of the Old Republic (By Drew Karpyshyn) where The Huntress sneaks up on a force user:

It was practically impossible to sneak up on a Jedi, yet Set hadn't felt her presence until she'd spoken.

Note: Set Harth was a male Human Dark Jedi active during the decades following the Ruusan Reformations of the Galactic Republic.

Answer 3

1. In EU/Legends canon, it is impossible to sneak on a Force user who is alert and aware of his Force-surroundings.

   Don't have the cites at hand, but we see numerous times a Force user detecting life-patterns of everyone in the near vicinity, including different rooms of same building.

   To escape that, special circumstances are requred: suppress the Force in general, (Ysallmri in Thrawn books), suppress the Force from another Force user (other Force users as shown in other answers).

2. In Disney/Lucas canon, it's most certainly possible as it happened.

   Most notably (spoiler :) Han Solo and Chewie in Millenium Falcon sneak up on Darth Vader in his TIE/Advanced above the Death Star and blow up his wingmate without Vader noticing they are there.

   Vader concentrating on Luke had something to do with that, but we see in other canon that Jedi are able to sense/track MULTIPLE persons at once, as a big picture, if they try.

   However, I would file that into "special circumstances" bin as well.