Paranoia
The first time Lestrade got into the dark car, he had imagined to continue their discussion on Sherlock's safety and involvement in the investigation, the one they had started in his office. Apparently, they had finished with the talking, and Mycroft had very efficiently explained it to him with his tongue down Lestrade's throat.
Now, countless other clandestine encounters in the back of Mycroft's car later, he never questioned his intentions anymore, followed Mycroft everywhere he wanted, knowing they would ultimately lead to Mycroft's big bed and unprecedented pleasure.
This time, utterly satisfied, his lips still tingling with the fervour of their furtive, almost stolen kisses, he got off the car and into his building. He put the key in and opened the door, finding himself assaulted by the other Holmes, who grabbed him by the collar of his shirt and pulled him in for a voracious kiss. "You're late," he said, but then, instead of punishing him, kissed him again.
Mycroft had made him forget that they had just wrapped up a case.
A bit of panic settled in, triggering paranoia because these two were Sherlock and Mycroft Holmes, and did he really think he could fool them?
Didn't they notice all the little signs?
They were usually so observant, so were they pretending or were they really so blind?