Check inside any decor. Most aquarium decorations are hollow, and depending upon how large your pleco is, they will shimmy themselves into any nook. My one sun catfish used to get underneath a fake tree trunk in my tank and live up inside it until I finally took it out because I was afraid he'd get stuck. Now I made some natural rock caves for him where I can peek into them and make sure he is there.
My one pleco, however, stays inside a hollow spot on the tree trunk and only comes out for food. It's a pain to try and lure him out, so I've given up and let him have his hidey hole.
~~Colesea