Quick question and I’ll leave ya all alone.

1995 6.5L diesel, long time member here, had it since new. Gave my son that suburban. Today he says it’s drinking the coolant, 3L at a time, starting today. Coolant low light keeps coming back on, it’s down each time. Didn’t dump on the ground, didn’t dump on the floor mat, no white smoke.... where’s it going?

You seem to have covered the bases - It's not leaking externally. Is this a dual-heat truck? If so, start looking at the floor and drain openings in the passenger footwell and the rear cargo area.

Just to be sure, check the transmission oil level and condition. You probably see where that is going. If that is all correct, and the engine oil is also not contaminated or overfilled, that only leaves the exhaust system as the exit path for all that coolant.

If it isn't leaking in the vehicle, that only leaves internal system leaks. As you indicated, those are susually evidenced by vapor/white smoke - Usually. A diesel can be a little different.