For some concepts, yes. Radical new approaches to things that give you an "Aha!" moment will be memorable.
But often you synthesize ideas too. You take your body of knowledge and produce a good idea from it. Some concepts are "obvious" when you've learned other, related concepts.
In those cases you wouldn't have an external source. After years on the job, you should have a lot of things like that. They may not be original ideas, people have come up with the same ideas before, but nobody told them to you.
I have this only for concepts on radically different topics.
If it's something about history, I probably learned it from Revolutions, because I don't read a lot of history otherwise.
If it's about cryptography, I probably learned it from The Code Book, because I don't read a lot about cryptography otherwise.
If it's about language features in Python, heck if I know where I got it from, I watch, read, hear about, and find new stuff about python all the time, it's my job.
But often you synthesize ideas too. You take your body of knowledge and produce a good idea from it. Some concepts are "obvious" when you've learned other, related concepts.
In those cases you wouldn't have an external source. After years on the job, you should have a lot of things like that. They may not be original ideas, people have come up with the same ideas before, but nobody told them to you.