The little exchange between Izaya and Simon that took place in Russian is something that I didn't think too deeply about until after I saw an English translation, which I thought it a bit too direct. Looking at the Japanese sub:
「錐は袋に隠せないぞ」We get something like:
"You can't hide a drill in a bag."
"Don't talk about ropes in the home of a hanged person."
Japanese bloggers have also been talking about these two little proverbs, which are apparently "Шила в мешке не утаишь" and "В доме повешенного не говорят о веревке", and they really seem to imply more than the translations let on.
The most common clarification for the first is that a drill will poke through the bag, perhaps destroying it. Assuming that they're talking about Celty's head, then the implication is that the head will reveal itself no matter where it is kept.
However, Izaya's reply is somewhat more confusing. As this blogger notes, it seems to encompass the idea that you "shouldn't rub salt into someone's wound." Yup, go figure...
ちなみに、"Шила в мешке не утаишь"「錐は袋に隠せないぞ」という諺の意味は日本の「嚢中の錐」のと似てい