Hidden machine stands for HM so any place there are hm's that's what your looking for. They are usually in a variety of places, but most are obtained from people in houses in certain towns, and you can only use them after you defeat certain gym leaders. When the gym leaders say "now you can use hm05 outside of battle" or whatever hm, you know that the hm was found most likely in that town, or the town before, or in between.
