He's not sure where they are. Well, he knows where they are geographically – he's in Canada and Sirius is in England – but he's not sure exactly where they are as people, as friends, as maybe something more.
The easy way Sirius' lips grazed his cheek once as they pulled away from an embrace just seemed natural – normal even and it seemed only natural that Remus should return the sentiment the next time they had an occasion to embrace – Quittich Finals when Gryffindor won 170 to 20 against Ravenclaw.
From there, things get a bit hazy, as it were. Remus can't remember exactly when Sirius' half kisses on the cheek turned into half kisses near his mouth and then a full kiss on his bottom lip. He doesn't know exactly when he started anticipating these intimacies or why he missed them so much when they were absent. And he definitely doesn't know what the hurried kiss in the shadows before they left for the hols meant and why Sirius turned around with a wild, furtive look in his eye before he was pulled away by James.
Remus stares down at the postcard in his hands. What to write when one feels like they're stuck in the middle of the ocean despite the fact that their feet are stuck firmly on land. He goes for a walk, takes off his sandals and lets the cool water lap at his feet. He squishes sand between his toes and steps over seaweed and thinks of Sirius. He clasps the postcard firmly in his hand as if it might fly away to Sirius before he gets a chance to write on it.
Finally he stops at a large rock that looks sort of like McGonagall's head sideways and sits awkwardly atop it. He pulls his quill out from behind his ear and turns the postcard over, the pretty Prince Edward Island sunset against his thigh. He wants the quill to write it for him, wants to know exactly what to say, how to communicate these feelings that seemed larger than the ocean that moved in front of him.
In the end, he writes 'Wish you were here'. It's so generic it makes him cringe a little, but it's the truth. Or at least as much of the truth that he's willing to bare on a ten cent postcard of a sunset he's never seen. He smiles a little, thinking of Sirius running up and down the shore in his Animagus form, a large black silhouette on the horizon. He smiles and kicks some sand before putting his sandals back on and tucking the quill behind his ear once more. He looks at his scrawl on the postcard once more before carefully tucking into his pocket and thinking, Remus Lupin, you are such a girl, trying to put feelings onto paper.
He dusts off his bum before walking back to the Bed and Breakfast he's staying at with his parents. When he gets back, there's a letter waiting for him, a small bit of parchment that says:
Miss you. Wish you were here instead.
Just above Sirius' name though, there's something scratched out that Remus peers at until he can make out a word. Love. It says Love. Remus smiles to himself before adding something quickly to his own postcard. He doesn't look at it again before giving it to his owl to fly over the Atlantic Ocean to Sirius.