low side service port
 

I am new here and have a quesion. I have topped off ac systems in my own car int the past with no problem, however a friend brought his 02 trailblazer over to top off and I could not find the low side service port. I like to feel I looked everywhere for it but I still could not find it. I followed the lines and only found the high side. Could someone who knows about this please point me in the right direction? Thanks for your help.