>>181 ありがとうございます

リンク先に以下の記述があります。
Feature Overview
When an Address Resolution Protocol (ARP) query is triggered for an
address that is configured with Network Address Translation (NAT)
static mapping and owned by the router, NAT responds with the BIA MAC
address on the interface to which the ARP is pointing.

The ARP queries are always replied to by the HSRP active router.
In the event of failure of the active HSRP router, upstream devices
will point to the new HSRP active router and will not have an ARP
entry pointing to the original active router, which may no longer
be available. Static NAT mappings must be mirrored on two or more
HSRP routers, because NAT state will not be exchanged between the
routers running NAT in an HSRP group.

これによると、やはりbiaでARP応答するみたいなんです。