Winston has committed a grievous offense by writing in his diary during his lunch break, and he wishes to conceal this transgression:
It was exactly the kind of detail that might betray you. Some nosing zealot in the Ministry...might start wondering why he had been writing during the lunch interval, why he had used an old-fashioned pen, WHAT he had been writing — and then drop a hint in the appropriate quarter.
This passage not only emphasizes the extent to which the Party sought to control the lives of the people, but also the way they sowed mutual distrust to that end. Winston fears not so much that the Thought Police will come and kick in his door, but that he will be sold out by some gossipy fellow Party member. There was no room in Oceania for people who thought on their own, and committing things to paper was thus a very dangerous act. Inkstains on one's fingers could be a dead giveaway. It may be, of course, that Winston is simply paranoid, but either way this subtle passage is a powerful demonstration of just how thoroughly Big Brother has insinuated his way into the lives of Party members.