Being the dominant fish in the tank (i.e., claiming a territory and holding it against all the other fish in the tank) doesn't make something predatory. The two things are totally unrelated. There are docile, schooling fish that are out-and-out predators (e.g., needlefish) and then there are incredibly aggressive territory holders that only eat algae (e.g., damselfish).
A plec will eat any dead animal. None of them catch and eat fish, except perhaps fish eggs and immobile larvae. If your frog has vanished, it may well have died, and the plec has done what comes natural and eaten the corpse. That's what catfish do.
That said, I'd look to see if its escaped (never underestimate the suicidal tendancies of aquarium animals) and also check behind things like filters. Often, things get wedged in the most unlikely places. I've even found fish inside filters!
Cheers,
Neale