Yes, he is probably just really scared and found a very good hiding place. I've heard Otos can be shy anyway or at least need some cover from plants or so. That's probably where he is right now.
Previously: 2x 10gal tank, 45gal aquascaped, 70gal aquascaped
Currently: 16gal FW, 40W CF, ADA Aquasoil Amazonia II/Tahitan Moon Sand, PH 6.5, kH2, gH 4