There should be a hotspot at the top end of the line. ("Should" may not necessarily the case if that 'line' is part of an object.)
With the arrow tool, select the 'line' in your elevation window ... your Info box should then tell you what it is.
Since it shows up in Elevation, it must be a 3D element - assuming you didn't accidentally draw a 2D line. What do you see in the 3D window at the top of your chimney?
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