You just noticed that !
As each plane goes in for a maintenance (paint) check the Logo is applied in the area between the main gear.
DARK BLUE background and WHITE lettering "DELTA"
I live just off Runway 4/22 here at MSP and I have seen this on more and more aircraft.
From what I understand this is easily applied requiring no scaffolding or special gantry platforms.
Any station with a paint shop can do this with little or no down time for the aircraft.