I did an experiment with a resistor substitution box and a voltmeter across the IDC. I magnet tested my detector and then began switching in additional resistance across the IDC. The LED stayed on with 5 volts across the IDC. The next step lower dropped the IDC to 3.7 volts. The detector LED went out.
If you have a voltmeter check the voltage across the IDC in standby state and with the detector activated. The LED could be turning off because the IDC voltage is low.
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Ah, then that must be it then. Next time I have the opportunity to test everything I will run a test with the voltmeter and see what happens.