Do nothing unless there's a need to do something. The air vents in the old houses I have lived in have always been high up in bedrooms - I like to restore them to improve ventilation. Never seen a low down room vent.
My guess is that your "patch" might be where an electrical fitting might have been or an access hole for some kind of service or quite possibly the exhaust vent for a gas fire. You do not say the size.
Tap around the patch and on top of it and listen carefully - you should be able to tell if there's a hollow underneath.
The only concern you appear to show is one of a different colour - when you next decorate it will all be the same.
Peccavi
January 2011
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