I would run the water for a couple of hours after turning on the water heater, then take another sample. If your on city water, it's probably just from sitting so long. If on a well, get all the info you can. If it's a real shallow well, hand dug well or a very old well that could possibly have holes in the casing letting in surface water, the coliform bacteria may be there to stay.
Coliform bacteria is generally from animal feces. So if your near a farm, have animals in the yard near the well, that could be the source, but not the cause.