When Robin walks in, he fidgets. He's not really one to remark on anything about a woman beyond, "She's hot", but there's something about the blue sweater that kind of brings out the blue in her eyes. And he knows about these things because, hello, clearly a man with fashion sense as awesome as his knows what he's talking about.
But Ted has a girl in his bedroom. Ted has a girl in his bedroom. And while he would usually be high-fiving for this kind of debauchery, it's Robin, so the high-fiving seems inappropriate somehow. She's going to get hurt. Somehow she and Ted have started this dance with her being completely oblivious to it but he starts to see affection in her eyes when she looks at him. They're getting there.
Lily wrings her hands, Marshall clears his throat. They all know what's going to happen. "Top of the morning to you," he mumbles. And later, as they shuffle into the kitchen, he files behind Marshall and Lily (who are still trying to eavesdrop) and thinks about covering his ears (he doesn't). But he tunes them out anyway. He never really was good at dealing with disaster (still isn't).