Sherlock first noticed the strange behavior in his mate when he couldn't find his plum shirt. He looked around and started noticing that other pieces of clothing of his had also disappeared.
Sherlock had been keeping his clothes in his room while he slept in John's room, as it left a floor of buffer between them and Mrs. Hudson for their... night activities. But lately John had locked Sherlock out of their room, Sherlock took a few days to notice as he had taken on a big case during that time that he had only just solved, and wanted to take his blogger out to dinner to celebrate and get some more time in with him.
Sherlock climbed the stairs, but halfway up a sweet, mouthwatering smell hit his nose, and Sherlock ended up taking the last half of the stairs two at a time.
"John-" Sherlock stopped. the bed had been dismantled and pushed against the wall, and the mattress seemed to be the base for the structure that took up the middle of the room. A flat circular piece of architecture with an indent in the top. it had maybe a six foot diameter, easily fitting on the king-sized mattress. The structure was built out of blankets and pillows, explaining where Mrs. Hudson's missing bedsheets were. The top layer of the 'nest' for lack of a better word, was created out of things that belonged to both Sherlock and John: blankets, sheets and an odd assortment of clothing.
In the middle of the indentation lay where the delectable smell was coming from, John.
"Sherlock," his pupils were blown wide. Sherlock had restrain himself from growling and taking his omega right there.
"What is this?" Sherlock wanted confirmation as to what his omega was building them.
"It's our nest," John whimpered.
"Then it's where we'll spend the rest of your heat," Sherlock could feel his pupils growing in size. This time he didn't resist his instinct and stalked over to the nest, taking off his shirt on the way, stopping just before the rim of the nest. An omega's nest was a very intimate space they guarded fiercely, and entering without permission could be the end of someone.
"Sherlock come on," Sherlock took that as an invitation and finished stripping before climbing into the nest with his omega, who was equally unclothed.
"Why didn't you tell me sooner?" Sherlock growled into John's neck, spooning him from behind, waiting for the preheat stages to end before mating to save up their energy and to make sure that John had produced enough slick so Sherlock wouldn't hurt him.
"I wanted to finish it before my heat and show you it as a place for us, but my heat took hold a day early," John admitted looking at his hands.
"It's perfect," Sherlock growled in John's ear.
"I think that you are far enough into your heat to begin mating with your alpha, don't you think so, little omega," Sherlock let his alpha instinct consume him, giving into carnal pleasures and going into rut, pulling his omega into full on heat.