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Combine them with a GPS sensor (accurate enough?) and track the area that they've covered on a map.

If that doesn't work, maybe some sort of paint drip system in the legs?

Or have them periodically drop seeds of some kind

Doesn't work very well in sand.

We do this in search and rescue work. It seems like a really good idea.

I wonder if there is a way to make a more cylindrical version of this, which might follow a more predictable and controlled path through a minefield.

With S&R being off by a meters or two generally isn't a problem. With landmines drawing the 'safe' line one meter too far to the left can be deadly.