if you used a soldering gun (?) the transformer reduces line voltage to create high current and this voltage is enough to stimulate Vf and illuminate with a common mode voltage and some stray capacitance to create a differential voltage.. When I desoldered them they emitted visible amounts of light.
probably so, it's a HANDSKIT Digital Soldering Iron.or if you used an ungrounded iron tip, the stray e-field is enough to induce xxx uA of current. (Although ESD protected irons only have 1Meg to PE gnd)
that was what I was wondering about. Thanks for the explanation.i.e. it is not a Peltier or thermal effect.