Police are appealing for witnesses after the incident at St Mary's Hospital in London Road between 2.30pm and 3pm on December 20.
The victim was visiting a patient and when they returned to their silver Honda Jazz the catalytic converter had been removed.
No descriptions of those suspected of being behind the theft were available from police.
A member of staff's car was targeted in a similar incident at Kettering General Hospital last month.
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There has been a huge rise in catalytic converters nationally. Catalytic converters clean up harmful gases before they exit the exhaust pipe.
The precious metals used in them are highly valuable.
Witnesses or anyone with information should contact police on 101 or Crimestoppers anonymously on 0800 555111.