As a technical coach, especially when I am embedded in a team, it is hard to overemphasize the importance of using the phrase "I don't know," plus "let me go find out." or "I'll ask someone who I think can help us."
Being able to say "I don't know" is an important skill for developers if they want to be on a high-performing team. Without it, the team risks creating an environment of fear and delayed learning. So when I say "I don't know ..." as a coach, I am modeling behavior that will help devs succeed. And adding the second phrase models taking initiative. While many, perhaps most, devs are good at finding answers to things they don't know, there can still be cultural and other reasons why a developer might not step out of their usual domain.
If you have coaching responsibilities, either formal or informal, and you don't say "I don't know..." maybe ask yourself if you are missing an opportunity to model important behavior.