Missing Prims

On a quarter sim I have a number of builds. I keep a reasonable eye on the prims and how they're getting used up. I have the standard 3750 allowance and when I check it tells me I've just under 500 left.

Thing is, when I select all of the objects in different parts of the plot and add up the numbers of prims in each I think I should have rather more than 1200 left.

I've done all the usual things; highlight objects, checked other people's objects, looked under ground, checked out the sky up to 2000m, and a friend has done the same for me and we can't figure out where the 'missing' prims are.

The sim has been re-started with no effect.

Can someone give me any advice on where the missing prims might be?

Was the selection of "all of the objects in different parts of the plot" done by surrounding the objects? (Sometimes an inadvertent drag-copy can leave duplicates at the exact location as the originals, so selecting by click will get one or the other, but not both.)

that's a good question, but the selection was a click and drag across the visible object, so I don't think its the answer.

When I first came into SL , I practiced rezzing objects on a friend's land. I didn't understand at first when to rez out a box or bag and when to just Wear. Months later it turns out that I had rezzed items inside of some of his prims. Jer had to go around and check all within that area. He was surpised at how many were hidden inside of some of his prims. I'm not doing that sort of thing anymore, lolol. It can't hurt to check for hidden items inside prims/objects.