Maris Ayin is a technical halachic term that gets lots of sloppy abuse. Maris ayin (it appears to the observer) can be a factor in determining halacha, but only where Chazal used it that way. For example, wearing a signet ring is patur aval assur according to some because the viewer may think that one is wearing the ring as a subterfuge to be able to use the seal. The Halichos Shlomo in Yom Kippur brings two halachos regarding shoes that seem to contradict each other, but actually very neatly show when maris ayin just doesn't apply and when it could apply.
- One who has a heter to wear leather shoes on Yom Kippur (for whatever reason; sores on his feet, for example) may wear them with impunity; in full view of any and all. No maris ayin here! (You may want to be a bit circumspect because of l'zus s'fasayim/crooked lips, ie, slander; but that is personal taste.)
- One may not wear simulated leather shoes in Yom Kippur because of maris ayin. Even at home alone in your sealed room, no dice and no simulated leather shoes.
His proof is fascinating. Whenever something is forbidden because of maris ayin, it is forbidden "afilu b'chadrei chadarim"/even in the inner chambers; ie, even when there is no possibility of observation.
Important aside: This is an important rule that has application all over halacha: once something is determined to be forbidden, the actual reasons that went into that decision are not usually relevant to the application of the issur. The fact that it was forbidden because of what people will think does not mean you can ignore the issur just because there is no possibility of observation.If one is permitted to due something that is usually assur because of circumstances, however, then one need not publicize it -- even if it is something usually done in inner chambers. For example, cooking on Shabbos for a choleh sh'yeish bo sakana/a deathly ill patient is permitted and one need not make any special efforts to publicize ones actions and their reason. That is, the action is permitted even in inner chambers and therefore it cannot be a case of maris ayin.
By the way, Halichos Shlomo also says that now a days the minhag is to wear very comfortable non-leather shoes on Yom Kippur; because, he says, we are all istanises (loose translation: spoiled brats) today. He pretty much calls 'em like he sees 'em.