I've had a few high readings on my own HRM, they sometimes occur in roughly the same places - part of one of my regular runs stays close to the railway for a bit, I've had it there and it goes when I turn away so I suspect the overhead power lines. Theres another location where it happens occasionally, no visible lines but maybe a buried one? Was there anything near where you were that could have explained it? It sometimes happens in other places, I think maybe the signal gets interfered with more than we think.