You wouldn't notice it at first. The evidence was far too elusive to be picked up upon at first glance. Their relationship would become a lot more real if outsiders were able to pick up on it; it would mean that it was much more serious than an occasional shag with a far too congenial morning after. That's all it was, they told themselves as they hid the proof in an effort to keep up the delicate fairy tale they had created. Just a drunk fuck, though they made sure to ignore the fact that they were both completely sober more often than not.
But it wasn't impossible to see the proof; all you had to do was take a more careful inventory of the situation. Take Arthur's apartment as an example: it looked completely innocent upon first glance, if you were to peek into his bathroom cabinet you would find an old tea tin halfway full of bobby pins and other various hair accessories that always fall out of Marianne's hair and become scattered around his apartment. He found himself collecting them without even thinking about it; when he found a bobby pin in his bed (and under the rug and in between the sofa cushions and in the shower) he would unconsciously pick them up and tuck them out of sight, though he always made sure to keep them where she could still find them. This was high on the list of things they didn't speak of.
The other details were somewhat easier to spot if you knew where to look. In the bottom drawer of Arthur's bedside table there was a bottle of Marianne's favorite hand cream. He told himself that he kept it because Marianne would throw an absolute fit if he dared toss out her precious lotion, but they both new differently.
And then there were the sketches. The doodles were the most obvious sign of his hidden affection, and just so happened to be the only sign she remained completely oblivious of. They were kept in an old stationary box in the bottom drawer of Arthur's desk. She had a habit of doodling on anything she could get her hands on and as soon as she was finished she would scatter them all across Arthur's apartment. Once she left he would dutifully pick them up and stash them away on the off chance that she would ever want to see them again. He knew that she wouldn't want them back, but the lie that she would was solely for his benefit anyway. Arthur adored the doodles even when he didn't enjoy the stories. Marianne illustrated children's books for a living and her latest doodles were filled with sheep that had various household objects caught in their wool. He thought the story was ridiculous but she loved it.
'You just don't get it! It's a classic tale about working to achieve your dreams!'
'Stop being so full of yourself, Bonnefoy. They're sheep who want to eat toast in bed. It's hardly great literature and that isn't even a real goal!'
Acknowledging any of it would mean acknowledging a relationship, and that was to be avoided at all costs. Instead they choose to bicker and occasionally sleep together; all the while ignoring whatever relationship they had. Maybe they would acknowledge it one day, but not any time soon. When the time comes, Marianne will be the first one to call an end to this nonsense. But until that time, they'll carefully preserve the fragile waltz of unacknowledged feelings and not very well kept lies.