Routers know how to figure out what route to use for a given IP. If they apply this algorithm to the source IP and find that the optimal route to the source is

Routers know how to figure out what route to use for a given IP. If they apply this algorithm to the source IP and find that the optimal route to the source is a different interface than it was received on, that’d potentially be a red flag. But if the optimal route to the source is the same as the optimal route to the destination, that’d be a huge red flag.