I don't recall any important safe there... but just to be sure I looked up in the official guide for you

It mentions only two safes in REPCONN Test Site. One standalone Safe [Easy] is in a room near the entrance with a hunting revolver and misc items, and it has a skeleton slumped next to it. The other is a wall safe in the Executive Office which has Carton of Cigarettes and "Wall Safe Items [Average]"
Do you want more specific directions to those safes? -- if you missed it you're not missing anything important except maybe your wounded pride lol
I spent a heck of a long time there chasing a ghost ... because I got REPCONN Test Site mixed up with REPCONN Headquarters (no, not the same place at all)