Say I'm on the road traveling, and I need to use a restroom. Which of the following two restrooms is preferable?
One in a restaurant that used to be kosher, but is now non-kosher.
One in a restaurant which has always been non-kosher.
Presumably if you're traveling on the road, your not likely to be recognized by your face as people will not know you. Keeping a baseball cap handy, can take away the question of "Mares Ayin" when the situation arises.
I think a more interesting question might be with places like Dunkin Donuts and Starbucks that are frequented by many frum Kosher keeping Jews for their basic coffee products that are deemed acceptable by many authorities. However, many of their other products are not. Do we suspect that someone might come to assume that all their products are kosher based on seeing you there?