LEDs can be very sensitive and so are your eyes. remember that the DCC signal/power voltage is actually flipping in +/- polarity several thousand times per second and that every wire and electrified rail has a small amount of capacitance to other objects. Charging and discharging that capacitance from that rail or wire to other objects, even if they are supposedly not electrified, will let current flow through a test LED to temporarily electrify the 'other side' with a repetitive amount of current to flow... a few hundred micro-amps perhaps, which can be great enough to dimly light up a sensitive LED so that your eyes can see it. The BD20 on the other hand supposedly requires maybe 8-10 milli-amps of current for track occupancy detection. Long leads of twisted pair wiring and/or the track itself can form enough 'leakage capacitance' and DCC leakage current flow to fool a current sensitive occupancy detector like the BD20. This why it is best to place a BD20 out by the track section to be measured and not use twisted pair wiring placed after the sensor.
Just for giggles notice in some situations just holding the other LED test lead in your hand forms enough of a high frequency antenna and power absorption to light up a sensitive LED.