I don't think psychic energy or magic energy count as electromagnetic, so at the very least you'd be able to see that (due to him using both psychic and magical abilities at the moment) with Seek.
Even under circumstances where nothing else is active, someone able to detect magic energy should be able to see him through those senses as Radiant Sight and Radiant Devourer are always active.
EDIT:
OOC: Lucifer doesn't hide. He simply doesn't allow those who don't share his vision the ability to bear witness to his form.