The docs also seem to indicate that 3mx3m is the smallest grid they can address, but at the same they use addressing in slums as a use case...who has that much real estate in the slums?

Hi - plus codes founder here. You can just keep adding digits and the area gets smaller. The way I think of it is:

+XX "house" (13x13 meters) +XXX "bathroom" (3.5x2.8 meters) +XXXX "chair" (87x56cm) +XXXXX "envelope" (22x11cm)

In Kolkata the NGO (Addressing the Unaddressed) are using +XXXX codes.

Wait a minute. How are Kolkatans with (probably) cheap phones getting their hands on coordinates that accurate when phone GPS is usually only good to within ~5m?[1] I assume the postal workers are getting high end equipment?

[1] "GPS-enabled smartphones are typically accurate to within a 4.9 m (16 ft.) radius under open sky" -- https://www.gps.gov/systems/gps/performance/accuracy/

First guess: Signs, maybe? You only need a few accurate surveyors and they could write coordinates in places. Use the cheap accuracy to get close then follow the signs.

Looking up the charity mentioned, they even have a nice photo of such a sign, so my guess appears validated: https://www.addressingtheunaddressed.org/services

Neat.

Thanks for clearing that up!