I have this headcannon about A Scandal in Belgravia that if John had seen Irene's phone he would've been able to guess the password a lot quicker than Sherlock. Because John gets these things.
John observed as Irene and Mycroft attempted to bargain for the information on Irene's phone. He knew Sherlock was sitting elsewhere sulking over Mycroft's words on the plane, but he wasn't in the mood to deal with his flatmate's strop. Watching Mycroft Holmes be outwitted was much more entertaining.
Idly, he reached out for the phone that was the cause of this whole mess and turned it on. He read the lock screen and then paused.
He glanced at Irene. Then over to Sherlock. Back to Irene.
His eyes narrowed in suspicion.
He pondered for a moment. Then another.
He looked back at the phone and waged an internal battle before finally giving in and shrugging. He typed in four letters, and then snickered quietly when he saw the home screen.
Both Mycroft and Irene turned their attentions to him, both with questions in their eyes. He slid the phone over to Mycroft, enjoying the brief look of incredulity on the man's normally blank façade and reveling in the fact he managed to put it there. "Well, Miss Adler, it seems we have nothing more to negotiate," he told the astounded woman.
"How?" she asked John.
By this point Sherlock had become aware that something had changed and wandered over to see. He was frowning, looking between the phone and John.
"It's not like it was brain surgery," John replied. "I observed the subject and deduced with my plebian mind. 'I am Sher-locked' was hardly a difficult connection to make, given the way you've been acting around him. Don't think I haven't noticed."
There was a wounded look in Irene's eyes as she regarded the doctor, and a flash of impressed in Sherlock's before it was gone. John stood from the table and stretched. "So now that we're done here, dinner?" he asked Sherlock as he gathered his coat.
"Starving," the detective replied.