No offense to Scotch, but that first line is a little difficult to understand. Is it that Dragon disappears when the RA is in Goa and is seen watching Luffy from afar? Or did Kuma himself see Luffy and then arrive at the conclusion that he's Dragon's son?
No it only means that Dragon hides from Luffy when the Revolutionaries go to Foosha island so that Luffy does not see him.
Dragon basically avoids Luffy so that they don't meet each other.
Maybe he does this to protect Luffy.
Kuma realized this because Dragon only avoids this territory and not others.
After seeing Luffy training hard in Foosha island, he connected the dots and realized that this was Dragon's son.