Do scatter plots ALWAYS have a trend line?

Oh heavens no. When there is no correlation between the independent variables and the dependent one the data points will scatter all over the graph with no bias whatsoever.

Wait let me reword that. When there is little to no correlation, you can still draw a line through the scattered data points, but it's math result would be close to zero.

no - There may be No co relation between the variables

http://serc.carleton.edu/mathyouneed/graphing/bestfit.html