Despite them being a no secrets type of couple, Will never knew that Nico had to listen to the sound of rain playing through the demigod safe headphones the Hephaestus cabin had developed to get to sleep.
They didn't have sleep overs, since the Apollo cabin forbade it and threatened to murder Will as necessary if he ever did, so they just sat with one another during meals and every once and awhile they would go to Nico's cabin and read books or watch movies, Will going back to the Apollo cabin when it got darker and the others were border-line about to storm over there and drag him back.
After a particularly bad mental health day, when Nico fell asleep, he accidentally shadow-traveled to Will's bed, headphones and all, causing the golden haired boy to fall out of the bed with his bundle of death on top of him.
He was, of course, startled awake and immediately noticed the headphones attached to his ears.
Nico explained, and apologized for waking him (and the rest of the cabin) up, and rubbed his eyes sleepily, sighing.
That's the only reason the Apollo cabin finally let up on their "No sleepovers" rule, not that the camp didn't already have one in place.
Not that that had ever stopped them, though. But they were responsible enough to save the big boy stuff till after mirage.
So don't get any ideas.