It's not that I need a round exhaust. I was just concern that it might harm the engine. The car was hit at the back. Car fax said minor accident at rear. I did inspect the rear and there was no sign of a hard hit. The rust did not alarmed me and that could be due to my area like you said

 

Carfax and Autocheck is a good source to review car history, but they only collect public data or data they pay for. When buying a used car it's always good to get a PPI. Find a local mechanic that knows about body work as well. If your RX was rear ended then you have you answer. Hopefully it was minor and repair shop deemed the the kink small and either left it the way it is now or pulled it out a little. If body and paint work was done right, I wouldn't worry about it much. Enjoy your RX, (-_-).