dead man body under tree SE has some outdoorman check (higher outdoorsman helps) and cant be found on character under level 24 (in code compares your outdoorman to random number between 24 and 299 and level compares to 23)
old warehouse (one with vertibird) cant be found under level 8 and has lower chance to be found if you level is under 48 (compares your level to random number between 8 and 47)
rest have lower chance to be found if your level is less then 24 (compares your level to random number between 0-8 [depends on which SE] and 23)
so basically best you can do is to be level 48+ and having maxed outdoorsman, nothing else helps
edit : also keep in mind that you cant find SE at zones which have no encounters