My origin was carefully concealed from me. Yet I've always somehow known on a subconscious level. You can hide this sort of thing from the child forever and never have them consciously suspect anything, but on some level the child will know something and just be confused and blame herself for feeling weird.
Here are some of the scattered hints I've had through the years of being donor conceived:
When I was 6 or so, I asked my maternal grandfather where babies come from. He said something to the effect of "Mom goes to a doctor and he puts the baby in her." For a long time, this was weird to me only to the extent that a man born in 1906 would say this instead of using a more traditional stork or cabbage story.
The weird part was how my social father reacted when I told him about this. He went pale. He was upset and angry. And he said "From now on, always ask me about everything!"
I interpreted this as being angry for my grandfather being dishonest about the facts of life. But his anger was still an overreaction difficult to understand.
When I was 7, I really wanted a baby sister. I was told this was impossible, without any explanations. Even afterwards, when I expressed this desire again, pointing out to them that I know my mother still has her period and that I know they have sex, I got no explanation at all beyond the customary "It's impossible" followed by an awkward silence and exchanged glances. No "We've been trying, but you know we have fertility issues" or "We don't want another child" or any rationalization at all.
When I was 16, I became very interested in the myths of ancient Gnosticism, particularly the story of the Demiurge / Ialdabaoth, the false god of this world, stealing the gift of life from the real Unknown Father, putting Adam on this Earth, and persuading him there are no fathers except him. I wrote a play depicting this myth. The character of Ialdabaoth was so easy and natural to create. He was a comical narcissistic pathetic little meanie saying things like "I'm your Daddy... there are no daddies but me... say Daddy" to the newly created Adam.
In the Gnostic myth, Adam eventually finds out his real father is the unknown God beyond the cosmos.
When I was 18 and became sexually active, my mother told me, in a whispering voice, not to think I won't get pregnant easily just because they had trouble conceiving. "Your father was the one with fertility problems. He didn't have enough living sperm." It was said with fear and guilt for revealing something no one was supposed to know, so it didn't even occur to me to enquire further. I tried to forget I knew as much, because I knew he'd be angry if he knew I knew. She was the one who took the "blame" for infertility publicly in front of others.
My husband remembers me saying, much before I was told by my aunt that I was donor conceived, that I think of my mother's family as my "ancestors", and not my father's family. In our patriarchal culture, this is really quite strange, and he asked me "How come?" I didn't know. I answered "I just do. I look like them, I am like them, I feel like I'm descended from them more."
When my children were born, I didn't expect them to look anything like my father. I didn't know why. He said once my second daughter looked like him, and I laughed - it sounded like a joke. It was impossible. I had no idea why, but the very notion sounded ridiculous. I rationalized it to myself as follows: "I look like my mother and nothing like my father, everyone knows that. So I guess I don't expect my kids to look like him, either. But genetics don't work that way and I know it. Traits skip generations. So why do I still expect my kids to look nothing like him?"
The fact remained: my kids look nothing like him, either.