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... quick summary, the "B+" output (2 terminals & wires that run in parallel to the battery) is simple, but the "D+" output is misunderstood by many. It is more than just a dash monitor. One must apply a small current there to initially excite the field and "boot up" the alternator. ... In my car, that bulb (maybe all 3 above) is sufficient to start it outputting at idle. If I remove the charge bulb, I must rev the engine a bit to get it to kick in. Where the threshold lies depends on your bulb resistance. My confusion started when I replaced all cluster bulbs w/ LED's. They didn't flow enough current to boot the alternator at idle, though it still kicked in ~1500 rpm. I could live with that, but am preparing this car for my clueless son who will have it out of town, so I put a regular incandescent bulb back for the charge indicator.
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What you are seeing is the result of residual magnetism in the rotor generating enough current to self-excite the alternator. Many people will install Delco alternators, which have similar external connections to the Bosch / MB alternator, with only one wire and will wonder why they need to increase engine rpm for charging to begin.
Ensuring sufficient charge at low rpm could be as simple as installing a resistor across the warning bulb circuit in parallel with the LED. Select a resistor with slightly less resistance than the bulb you replace and you should have enough voltage drop to operate the LED.