Is there a way to locate a receptacle closed behind drywall?
The code states that the outlets need to be no farther apart than 12' and within 6' of a door. If you had more than what code required, that makes it a little more difficult. If you find one at the door then go 12' from there, literally measuring the wall length you should find one at every 12'. If you have a space larger than that you may have a covered receptacle. Your building plan should reflect this, referring to my comment about code requirement. Or it should show how many you have, if it has more than code would require. I presume the electrician had something to go by to reflect your needs.

With that out of the way, and you have a spot you suspect there is a covered receptacle there, use a straight edge, or anything with a straight edge that is 4' or maybe longer. Run it at the level you suspect where the outlet is and if there is a covered outlet, there will be a noticeable hump in the wall. For the screws will draw the drywall pretty well to the neighboring studs but obviously not so where the covered outlet is. If you find one, the highest spot will be the outlet.