I have 3 objects on my land (in Alope) that are all owned by me but they are invisible, and unselectable - and therefore impossible to remove. However, Scan-Foo reports:
Scan-foo: Information about 'Lace Panties and Bra':
Scan-foo: Object key: 0feea4f0-9571-3428-4a52-03c697528321
Scan-foo: Located at: <70.31111, 176.17787, 501.08875>
Scan-foo: Angle: <-0.00000, 0.00000, -0.00000>
Scan-foo: Rotation: <0.00000, 0.00000, 0.00000, 1.00000>
Scan-foo: Velocity: <0.00000, 0.00000, 0.00000>
Scan-foo: Owner: You own this object. (599a4f4d-8792-415a-811e-e60b4da43980)
Scan-foo: Information about 'Lace Panties and Bra':
Scan-foo: Object key: a730b437-8490-4a15-12bf-58c25aca6250
Scan-foo: Located at: <72.30238, 170.39798, 501.08875>
Scan-foo: Angle: <-0.00000, 0.00000, -0.00000>
Scan-foo: Rotation: <0.00000, 0.00000, 0.00000, 1.00000>
Scan-foo: Velocity: <0.00000, 0.00000, 0.00000>
Scan-foo: Owner: You own this object. (599a4f4d-8792-415a-811e-e60b4da43980)
Scan-foo: Information about 'Lace Panties and Bra':
Scan-foo: Object key: 34540ebe-ce54-8294-8fd1-682367b312b9
Scan-foo: Located at: <72.91051, 179.68010, 478.73526>
Scan-foo: Angle: <-0.00000, 0.00000, -0.00000>
Scan-foo: Rotation: <0.00000, 0.00000, 0.00000, 1.00000>
Scan-foo: Velocity: <0.00000, 0.00000, 0.00000>
Scan-foo: Owner: You own this object. (599a4f4d-8792-415a-811e-e60b4da43980)
...they are in fact - despite their misleading name - all ground, grass, objects. It's my guess that they cannot really exist at the height they were placed, and got stuck in some kind of limbo. Someone from Live Help also took a look and concluded their visit by advising me that the 'Grid Monkey' would need to remove the objects.
I don't think I've ever met a 'Grid Monkey' but I'm hoping that someone with better Deleting Powers than me can dispatch them to the Prim Graveyard - I don't need them to be returned.

Thanks!
[Also Bug Reported as: #280916]
Added: Thank you to all who helped sort this out... and especially to Kona the Bug Hunter.