Is it possible to see a Teredo IPv6 address in IPv6 neighbor cache? Is it possible to see a Teredo address (from `2001::/32` block) in an IPv6 neighbor cache? For example, in the case of Junos, in the output of `show ipv6 neighbors`. As much as I tested, this is not possible because traffic leaving the Teredo client is already encapsulated in UDP/IPv4.

You won't see Teredo addresses in the neighbor cache, because Teredo creates a virtual point to point link and therefore has no neighbors. A neighbor is another device (which, for this purpose, speaks IPv6) on the same Layer 2 link. But a Teredo connection has no such devices sharing its virtual link.