Maybe it died and the pleco ate it? Did it seem off and have you looked on the floor, a 2-3 inch fish could easily jump out of the feeding hole of it wanted to
The pleco is only small, about 4cm. If it did, surely it would take longer then a couple of hours to eat it? I'll have to take a photo later when I get home but the feeding hole is right in the corner, and there is a rock background covering a bit of it...so I'm not sure. I'll check the floor when I get home but I feel like my doggos would have found it by now