There are several different trespass laws and some different requirements for different situations. Posting is required for some situations, but is not required for cultivated land or fenced land. Posting is not required if the landowner or agent asks the person to leave (see Idaho Code Section 18-7008(A)(8)).
Idaho Code Section 36-1602 (part of the Fish and Game code) prohibits hunting fishing or trapping on another person's land without permission if the land is cultivated, posted or enclosed (fenced). http://www.legislature.idaho.gov/idstat/Title36/T36CH16SECT36-1602.htm
Idaho Code Section 36-1603 (part of the Fish and Game code) prohibits entry of another person's land to hunt, fish, trap or retrieve game if the land is cultivated or posted (by sign or orange paint). http://www.legislature.idaho.gov/idstat/Title36/T36CH16SECT36-1603.htm
Idaho Code Section 18-7008 (criminal code) identifies several types of trespass including refusing to leave when told to leave by the landowner or agent, and entry of another's land which is posted by sign or orange paint. http://legislature.idaho.gov/idstat/Title18/T18CH70SECT18-7008.htm
Idaho Code Section 18-7011 (criminal code) also prohibits entry of another's land which is cultivated, enclosed or posted with signs or orange paint. http://legislature.idaho.gov/idstat/Title18/T18CH70SECT18-7011.htm