You have a point gil Turner, but in my situation finding runs is difficult...
My experience has been much the same. Unless you get out there right after the ice is made and it made good clear ice then you can find the runs. The rest of the time, forget about it.
Getting back to the original post I've tried it under the ice. This was on a small river/creek, maybe 20-25' across and up to 8' deep. No bubble trails, all the rats are living in bank dens and there is about 6-8" of ice. We baited the conibears with carrots and then suspended them from a stick laying on the ice.
Results were disappointing but the method may have been substandard also! The rat population was getting hit hard by a disease, tough to say how much that affected the catch and how much of it was the method. I plan on trying it again sometime, just have to wait for the rat population to rebound.