There were a perfectly logical reason why Tonks hadn't woken Charlie earlier in the day. And that reason was... she hadn't been there to wake him. See, once he'd fallen asleep last night, she'd sneakily taken the Floo to spend the night with Remus. As much as the whole sneaking out part was kind of fun (it reminded her of the times Charlie would come visit her in the summer between school years and they'd sneak out together in the middle of the night), it also made her feel guilty. She wanted to tell Charlie, she really did. But until she figured out how to get him off whatever he was still under the influence of, it wasn't safe.
She'd planned on sneaking back before he woke up, but had overslept, and then gotten distracted with giving Remus a proper good morning greeting, and by the time she finally managed to convince herself to get out of bed and put her clothes back on and head home, it was past noon.
When she stepped out of the fireplace into the living room, there was a smile on her lips as she brushed ash off her jeans and hummed happily to herself. Only when she glanced up did she catch sight of Charlie standing by the couch. Blinking, but unable to keep from smiling, she said, "Oh. Wotcher, Charlie. Sleep well?"