It's not a micro, but there are microcontrollers that are comparable in size. An AVR ATtiny20 is available in a 1.5mm x 1.4mm package. That's just about a 2mm^2 footprint, it's totally insane.
And not just itty ATtinys, for example MAX32660 is full fat ARM M4F core in 1.6x1.6 mm package. It is the sort of thing you could hide pretty much anywhere.
