How John would ask

Sherlock to marry him:

He’d change his laptop password to Sherlockwouldyoumarryme. Sherlock would be trying to crack it for days/weeks before hacking into it when John refused to tell him or change it again.

So sherlock, wondering what could possibly be the new password, as soon as he hacked into it, he would go to the settings to see the password. John would come home to see sherlock, mouth open in shook, the would look up to see John holding a set of wedding bands.