Searchmap is an option that uses the map extent of the map that would be created given an extent and size for search purposes. It's useful for creating imagemaps from query output where you need to match the imagemap to a MapServer generated image. Looks like it broke between the 6.0 and 6.2 releases which tells you how much it's used - I may be the only one. I'd propose fixing in 6.4 and 7.0.
Looks like the option and associated computations are fine, so the issue is in the template code - specifically related to msTransformShapeToPixelRound(). Turns out that a small change in that function to fix a loop counter exposed bad logic in the shpxy tag processing for points - the conversion from map to image coordinates was occurring twice - whoops.
Fixed a bug (see #5157) where converting points from pixel to image c…
…oordinates with the shpxy tag was happening twice.
Fixed in 6.4, 7.0 and master.