I just saw another one wedging himself under the heater, but this time I watched until the heater light came on. The pleco did not move or seem to react in any way, but I noticed that it was not under the part with the metal coils, so perhaps it wasn't very warm where he was. I guess as long as I don't see any injuries, I'll try not to worry about it.
Thank you for being so reassuring, everyone. I really appreciate it.