I think that it depends on the backpack. If you were walking down the street with a heavy laden 80L expedition pack...you might become a target to be mugged for food, water, tools, etc. If you had a courier bag or a book bag, then you'd blend in fine.

I've been contemplating moving my BOB to a book bag for this very reason...unfortunately it's full of pockets and notebook sleeves and water bottle holders so fitting in the big stuff I'd want to bug out with isn't that practical.

Worse yet is the other end of the spectrum. I know people who think looking 'tactical' = being prepared...in the case of a bug out situation those are the people who are going to have a hard time blending in.