There are two important logical results that suggest there are unknown truths. The first are Gödel's incompleteness theorems. These imply that there are mathematical statements that are "independent" (i.e. neither provable nor disprovable) from the axioms of arithmetic. This means that there might be mathematical truths we can't ever prove.
The second, less well-known logical result that might interest you is called Fitch's Paradox, or the Paradox of Unknowability. The conclusion of this argument is straightforward: There are some unknowable truths.
The interesting thing about Fitch's paradox is that if it is correct, then some important substantive philosophical theses about the nature of knowledge must be false. For instance, Kant thought that just what it was to say that something was "true" was to say that it is knowable by somebody.* Obviously if there are unknowable truths, then that can't be right. Fitch's paradox remains controversial.
That should be enough to get you started.
*I'm not an expert in Kant, so don't just take my word for this. But this is the standard "kantian" position among anti-realists today, whether it is fair to impute this view to Kant himself or not.