Unless it is told by the front desk when you checked in, it is an amenity for diamonds, I wouldn't take anything at all. I mean don't front desk usually tell you that kind of things like perks ?

Usually these kind of pantry is right next to the front desk (very common to HGI, HI, and HS, and those I saw in Hilton, DT), as whatever you take then you are going to pay right to the front desk. (there is usually an open space or hallway connect to the front desk from the pantry) Yes, most of these pantry require your room key to access it, as they don't want when late at night, the front desk person step away a little bit, someone would walk in the pantry quietly and take something from there without paying and leave. Usually they would put those toiletry items that are complimentary in the pantry as well, like one time use toothpaste (those tear on the side packet ones), toothbrush, combs, and shower cap, occasionally maybe sewing kits as well.

I would definately ask what is free base on your status, instead of listen to one of your FTer friend, as it just seems too good to be true. I am not judging anyone, but I have seen there are cases when I was at the pantry, no one at the front desk (they step in the back office), I could have taken something and leave without anyone notice, but of course I wait til the front desk come back and pay for it.