Basically this. Whether you have feelings for them or not, finding out someone likes you can be embarrassing or flattering (or parts of both). That becomes even more likely, and complicated, if you're acquaintances or friends with that person.
We'll have to see what happens next, but looking over the manga, I don't think the signs are great for Miku, unless she really is the kisser (I think it's Nino). He had to have known about her feelings by the time of 74, but if he was in love with her, you'd expect more than the rather muted reactions he gave to the fake Miku's disclosure of Ichika's feelings.
In 72, when Yotsuba prods him about love, Nino and the kisser's face comes up. Those flashes are given to us from his perspective, so if he was aware of Miku's feelings by that point, it's really damning that she didn't appear in his memory. Heck, if he merely had feelings for her but wasn't sure if she did, you'd still expect her to show up. He's not sure who the kisser is, either, so of course he's considering her separately. With that said, the end of 71 really implies his suspicions that it's Nino are stronger than his suspicions that it's one of the others, even if he doesn't really know yet.
But we will have to see how everything develops from here. I'm fine with a 2,3, or 4 ending, but I still say it's 2 based on what I've seen.