(Reno, NV, USA)
I don't think it's a mystery at all. If they were here prior to the flood, then why couldn't they be here again after the flood. The nephilim after all, as explained in Genesis, were not angels.
They were the half breed offspring of angels that left their domain and travelled here to take earthly women for themselves. The bible discusses how God took those angels that left their natural domain and has locked them up for the time of judgement. So if it happened once, why couldn't it have happened again, after the flood?
There's no doubt that they were all destroyed in the flood, and in fact, it's probably the dead Nephilim's disembodied spirits that are whats here today that we think of as Demons. Demon's are not fallen angels, they are another type of evil spiritual being that, according to the bible, must reside inside of another body.
The New Testament eludes to them not being able to dwell outside of some other body, whether it be human or animal. It also has been said that their wickedness revolved around sexual sins, as their fathers deviated from Gods plan in a sexual manner with human beings, and thus the demons that are left behind like to tempt and deceive using sex. (this is just speculation, but it does make since)
So Satan's ultimate goal is to rule the earth and to trick everyone into believing that he can be as powerful as God, and this includes angels. So Satin had 400 or so years, from the time the Jews entered into Egypt, to try and populate the planet with his own form of life, a half breed human and fallen angel species that, according to the bible, were fierce and wicked. What a great way to try and attempt to keep God's chosen people from taking over the land...populate it with giants.
So my thinking is that if angels could have come before the flood and had relations with earthly women, then why couldn't they have come again after the flood and done the same thing?
And as God wiped them out with the flood, his plan for the Nephilim 2.0 (as Ill refer to them in Numbers) was for the Jews to wipe them out when taking over their promised land, hence why He told them to leave none of them alive.