

Turn signal or not, you shouldn’t be pulling out in front of them. You should be assuming they are turning into a driveway after the intersection, or that they mistook your intersection for the next one down the road, or that they left their turn signal on from a lane change 5 miles back.
Turn signals are lies until conclusively proven otherwise.
They theoretically could come from tap water. Reservoirs of treated water can be open to the atmosphere, and accessible to wildlife. Chlorine and dilution mitigate most of the risk, but not all of it.