Iris wasn't trying to sneak. She was trying to be politely quiet. However, that wasn't exactly a skill set she was born with. Oh, she knew when not to talk, but try to walk without making a sound? To try to not cough, clear her throat, or bump into anything in the dim temple... that she was not skilled at. It wasn't like she practiced that or anything. Not even with her son.
No, when Pothos was a babe, Iris make sure she and Zephyrus made their normal amount or ruckus if not more at all times when the boy was trying to sleep. It conditioned him to not wake up at every little sound and there were all sorts of sounds that went on in her temple at night. Things she didn't want to have to explain to a toddler that was roused from sleep.
So, while she was trying, Iris had the suspicion she was failing. “Hypnos,” she called quietly. “I come to you on behest of Hera. She has a request of you.” She started very formal as it was her goal to get in, make Hera's demand, then get out. Not at any personal disgust with Hypnos, his home or the Underworld. She just didn't fancy lingering about when there might be gods less pleased to see her meandering in their world.
Like Moros. There was one deity she did not want to encounter in a dark corner of the Underworld.
With her golden wings folded carefully behind her back, she waited patiently for a response. Hopefull there was one. Hopefully he wasn't sleeping because Iris really, really didn't want to have to try to wake him up. Normally, if that were the case, she'd just leave and come back at a more opportune time. But Hypnos sleeping was just Hypnos doing his thing and Hera wasn't like to tolerate a delay because her messenger was too polite to interrupt a god of sleep from.. well.. sleeping.