It was a built-in light as part of the left-hand landing light. As per B-25C/D Service Manual it's a B-3 type light (don't know exactly what type is this; the landing lights are type A-9).
Can be seen on the photo below and on earlier a/cs as well.
Passing lights, in red, were additional to the nav lamps, to aid in identification of the port side of the aircraft, allowing head-on traffic to pass on the correct side.
The Accurate Miniatures 1/48 B-25B representing the "Ruptured Duck" is now complete! It has been mounted to the deck and tied down. I used Berkley 50lbs test braided fishing line to simulate the ropes. Then I used two tweezers to make the knots. Finally they were then painted with Tamiya Buff (XF57). Once it was mounted I used EZ-Line to rig the antenna lines. Aside from the issue with the decals I really liked this build. The kit is well detailed and the fit was very good. It now sits on the display shelf while I decide what the next build will be.