As the other members point out, the likeliest answer to your observation is due to light shining on the contacts.
Because the circuit is not yet complete and power is not yet applied, there is no current flowing through the circuit, to cause a 'flickering' as a component which consumes current is attached.
What has probably happened, is that you didn't notice the 'shine' from the contacts beforehand and only noticed it when the shadow from the LED legs passed over them.