People come here and ask, “is so-and-so a mortal sin?” And the answer is always the same (no matter what so-and-so is). The answer is: maybe.
But, from what you describe, I think the likelihood of mortal sin is remote.
The exact nature of your question is not entirely clear, but let me construct a hypothetical example based on some of the things you said:
Suppose I open a door and, unknown to me, a woman is getting undressed in the room beyond, and I see her nude. Opening a door is not a sinful act. But, if it’s a bedroom or bathroom door, there’s a reasonable chance that someone might be undressed on the other side. We might not KNOW if someone is there, but if we don’t provide adequate warning then some part of us might be hoping someone is.
In this case, opening the door is certainly a sinful act. We knew it was a bedroom door, and we had some hope that someone was behind it. Is it a mortally sinful act? Maybe. It depends on how big of a part of us was hoping to find somebody there.
The door to my study is usually closed (to keep the cats out - they destroy stuff). It’s a study, so there’s no reason why someone should be undressed in there. OK, my example is getting kinda nutty, but suppose somehow I opened the door and there was a naked lady standing there. Forget about the how and why. I’m looking at a naked lady. I see that she is pretty, has a nice body, and is sexually attractive. Am I lusting? And, if I am, is it venial or mortally sinful?
I have no culpability for seeing her naked in the first place. But that doesn’t mean I can’t be culpable for any lust that results.
Noticing that someone is attractive (even sexually attractive) is NOT lust. Lust is what happens AFTER we notice. Do we fantasize? Is it a fleeting thought? That’s probably nothing more than venial sin. Is it a consuming desire? Do we act on it? Now we’re getting into territory where mortal sin is more probable.