Messages you didn’t know about tend to be rather rare when using OTP applications.
Because OTP behaviours pretty much expect you to handle anything with some clause in handle_info /2, unexpected messages will not accumulate much.
However, all kinds of OTP-compliant systems end up having processes that may not implement a behaviour, or processes that go in a non-behaviour stretch where it overtakes message handling.
If you’re lucky enough, monitoring tools 6 will show a constant memory increase, and inspecting for large queue sizes 7 will let you find which process is at fault.
You can then fix the problem by handling the messages as required.
[6] See Section 5.1
[7] See Subsection 5.2.1