He gets home to find Lucy in the bath.

Natsu left Fairy Hills in a very good mood. On the way, he encountered Happy, looking all scuffed up with a lump on his head, and sporting a black eye. "...whoa," was all he could say at the sight.

"Is that all you have to say!?" Happy snipped.

"Sorry... What happened?"

"...I landed in the pound... The dog pound," Happy grumbled. "Wouldn't have been so bad if they were still in their cages, but it was their "play time," meaning they were out and interacting with each other..."

"Oh boy... They must have been excited to see a new chew toy," Natsu chuckled. "You really ought to learn to keep your mouth shut."

"Maybe..."

"...you're not going to, are you?" Natsu deadpanned.

"Nope!" Happy said in a way too cheerful tone as they reached the front door of Lucy's... Now their place. He turned the handle... Only to find it locked.

"Oh right... Lucy has yet to give us a key," Natsu said. "Ah well, guess we're getting in the normal way! Breaking and entering!"

"Aye!" Happy agreed.

...

"...so THAT'S how you've been getting in!" Lucy shrieked as Natsu and Happy clambered in through the bathroom window... While she was taking a bath.

"...well, it's one of the ways," Natsu chuckled, and took a moment to admire Lucy's form in the tub. Lucy just sighed and face palmed, not bothering to cover up. He'd seen it all and had a power over her anyway... And at this point, it was starting to turn her on, not that she was ready to admit that...