Actually this seems very plausible. Seals aren't just plain ink or whatever someone might assume it is. They're very clearly chakra infused, and they don't necessarily stay completely visible at all times. See naruto's stomach. Only activates when there's chakra there to activate it.
We also know that chakra itself never disappears unless it is expended or released in some way. See minato and kushina being sealed inside naruto.
Also note that nobody who met Naruto knew any of this sealed/hidden chakra was on/in him. (Except for oro, and we know why).
With minato there to send chakra to the seals, I could see them still being there and being re activated.