A simple moment that is set during the Water 7 Arc. After Usopp leaves Sanji finds part of the crew on the roof instead on in their rooms. I noticed how Zoro was sitting behind Luffy a few feet away with the air of somebody who's been there all night. This can be seen as a continuation of 'Do Not Swallow Your Tears' since Zoro and Luffy are sharing a room(Get your minds out of the gutter!). Enjoy.
When his captain, after a couple of hours of restless tossing and turning, finally rolled out of bed Zoro was already up and alert-though his eyes remained closed. The swordsman waited till his captain quietly closed the door behind him and when the door knob turned back into place, he sighed-loudly.
There was nothing Zoro could do. Usopp had left and Robin was missing, but both happenings were the results of their own choices. Zoro could drag Usopp back and he could find Robin sooner or later(most likely the latter), but that wasn't what was important. They had left on their own accord, and the only way to fix this mess was for them to come back on their own accord.
All of them knew this, and Luffy knew this. There was no straightforward reason for what had happened and there was no easy answer for what was going to happen. It was left to Luffy to figure out their next move, their tomorrow. Of course he was human; he would want to grieve for the past events, but more importantly he was captain. He'd have to push his feelings behind, lock them down just until he could make sense of what to do. What they would do. And it was a harsh and lonely road but it was the one Luffy had to take, one Luffy had to stand alone, straining his neck to look ahead. Luffy had to move on.
And there was nothing Zoro could do.
All the same, Zoro picked himself up, slipped out to the roof and noiselessly settled down(The stars could laugh at him all they wanted but the room was stale and hot and Zoro wasn't planning on sleeping anyways). He preferred the high roof and open sky, just in case the marines decided to sneak up on them. He preferred the hard and cool concrete floor, just in case the soft bed lulled him to a security and sleep that they didn't have the leisure to enjoy, not yet. And he preferred where he sat, beside the bell and behind the clock tower, just in case his captain needed the strength to move foward by looking back.