UDP Scans necessary to identify IKE VPN services running on machine
These are really good sources that go into more detail
https://angelica.gitbook.io/hacktricks/network-services-pentesting/ipsec-ike-vpn-pentesting
https://infinitelogins.com/2020/12/08/enumerating-ipsec-ike-isakmp-ports-500-4500-etc/