ICMP reply from the firewall instead of endpoint destination

[000000]

Hello everybody,


What could cause ping to respond from a different IP?
When tested from source, the response message of the ping command is successful and it's coming from the PaloAlto firewall, not from the destination IP.
Where and how can I verify on the PaloAlto if this is expected or not?
What setup can cause such behavior on the PaloAlto?
Need to mention that the destination is RPC server to which the source can't connect, even though there is sucessful ICMP reply from the PaloAlto firewall. 
Source and destinaiton are on different networks.

Thanks!

[reaper · Accepted solution]

Incorrect implementation of NAT could cause the firewall to assume 'ownership' of IP addresses through proxy-arp

This typically happens if the destination in a NAT policy is set to a subnet

[000000]

Indeed, it was found that there is a NAT which causes the reply from the firewall. 
The traffic in question only returns through the firewall and hits NAT rule on it's way back.
However that traffic does not go through the firewall on it's way out.
ICMP is not affected by it, but any TCP will be dropped because breaks the 3-way handshake.