It was a white disc that moved along the displayed road ahead of the arrow that marked our position.
It took me a little time to work out quite what the issue was (it carried on for about a mile), but we finally worked out that it was the GPS system's way of pointing out that we were proceeding north on the south-bound carriageway. Normally GPS doesn't have a fine enough resolution to catch this sort of error, but we were using the Blackwall tunnel, and the two bores run a fair distance apart.
So next time we see this, we'll know what it means.
(There are roadworks affecting the Blackwall tunnel northbound bore. So that tunnel is closed at night. That late, more traffic wants to leave the Greenwich peninsula than to enter it, so they use the southbound bore for northbound traffic. This is quite a good idea when 15-20,000 people are all pouring out of the O2 arena - there will be hundreds or perhaps thousands of cars wanting to cross the river to go home. And yes, I was puzzling out the map while bellinghwoman was the driver.)