I burned 2 Thermacells at a time in Quebec last spring. The result of that hunt is the bear in my avatar. I saw numerous other bears at the site that week too.
Honestly, most bears are gonna know you are there because many of them approach the bait or feeding areas down wind to avoid bigger bears. A number of them really don't care if you are in the vicinity as long as you don't move quickly.
Some bears are spooky, but many of them will know exactly where you are and come to the bait anyway. They see you as subordinate if you are up a tree. Just some observations on my part. We went 12 out of 15 (coulda been 15 for 15) on bears last spring and I think every guy there used a thermacell.
Your mileage may vary, good luck!
-Charlie