What are the chances of septic arthritis coming back?

Unlikely: Appropriately treated septic arthritis ( both proper antibiotic and length of treatment) should be curative. Unless a new bug has entered the joint it is unlike;ly that it would recur. If the original infection were not totally treated it can return.

It's always possible: Bacteria can remain dormant for years so its always a possibility. If you've had it in the past, at the first signs of possible return, get to your doctor that day.