Maris Ayin is even “b’chadre chadarim”, you can be locked away where it is imposable for anyone to see or know who you are there; but Maris Ayin applies, as if someone did see you.
When someone sees me in a treif restaurant, they may say, “If that frum fellow with a yarmalke can eat there, then it’s ok to eat”. They don’t know that I’m only using the telephone or the bathroom. However, if someone sees me without a yarmalke in the treif restaurant, he will know something is not right and is not likely to be mistaken.
You know what the most popular hechsher is? It’s: Everyone Eats There.