Theremin
You are correct, it was a theremin.
"While not enjoying the wide use in classical music performance for which it was originally designed, the instrument found great success as the 'eerie' background sound in countless motion pictures, notably, Spellbound, The Red House, The Lost Weekend, The Spiral Staircase, The Day The Earth Stood Still, The Thing (From Another World), The Ten Commandments (the 1956 DeMille film), Ed Wood, Mars Attacks!, and Forbidden Planet. The theremin is used as a melodic solo instrument, rather than as a sound effect, in the soundtracks of Hellboy, Bartleby and Monster House. The DVDs for Ed Wood and Bartleby both contain short features on the theremin." - Wikipedia