No real hunch. Like some here we just thought all kids were smart little sponges.
We were once sitting at a wedding reception with our probably 3 yr old son beside a lady we had never met. She kept asking him all kinds of questions and he would elaborate on his answers. She tipped us off.

Looking back we kind of knew I guess but we are a pretty nerdy group and he just blended in.

When he was three he was always wanting to read parts of the books at bed time. Anywhere we went he had to read everything at the library or museums. Every morning he would read the nutritional labels on boxes of cereal and trying to figure the percentage values for a full day of eating.

Again around three maybe four we went to a pool party and all the kids were swimming and the adults were sitting around with their feet in the pool. He pulls in several life jacket and was reading type code and warnings on the tags. Asking questions about certain words he was sounding out and asking questions about drowning. The others were looking at us like we were the most crazy parents, almost like we were torturing him. I remember just shrugging.