Cost explorer will help you dig into the usage type and resources used.
here is the cost csv dump in gist https://gist.github.com/manish-2014/363482a08078e346238ef633...
I have also dumped my cloudformation stack template. I am 100% convinced it is the IPV6 egress , which is supposed to be serverless and hence a surprise as I did not do much with this VPC in December
Check the usage type in Cost Explorer; if it’s under ec2-other, that disambiguates it.
IPv6 egress costs the same as IPv4 egress; the IPs don’t.
As per this document, An egress-only internet gateway is for use with IPv6 traffic only. To enable outbound-only internet communication over IPv4, use a NAT gateway instead. For more information, see NAT gateways.
Pricing There is no charge for an egress-only internet gateway, but there are data transfer charges for EC2 instances that use internet gateways. For more information, see Amazon EC2 On-Demand Pricing.