Black dot in 'confirmation requested' symbol?

Hi all. Probably not a massively important thing in the big scheme but I noticed today that a couple of my contacts, for which the grey 'confirmation requested' symbol is present, have a black dot in the centre... it's not in the legend.
Anyone know what it means?

 

Hover your mouse over the dot and you'll see an explanation. The dots are usually grey or blue. Either can appear to be black depending on your screen/monitor settings.

 

They are a hidden micro dot message for KGB sleepers Comrade.

 

Now where did I put that cipher?

 

This is the thing - the hover just says 'confirmation requested' like the usual grey dot... but these two definitely have a black dot in the middle. Very distinct from the majority that don't.... so it ain't the screen either

 

Noticed the same thing. Hovering over grey dot or grey dot with black center gets the same response.