Answer: This label is used for materials with gamma and neutron emissions This label was specifically used on containers shipped by aircraft, and that reference is from the late 50s/early 60s. Any red label is going to be Group I or II, which as stated above emits gamma rays, neutrons, or both, and is potentially harmful even with the box closed. OP's lid came from something shipped by air, but it's interesting to note that on ground shipments of the same class of materials (like with the label variant I found in my first edit), the trucks themselves were required to be marked "CAUTION" or "DANGEROUS - RADIOACTIVE MATERIALS" on the sides and rear of the vehicle just to haul this kind of material.

Post author update: As of today a 3 man team from the state of Utah department of environmental quality, division of waste management and radiation control, uranium Mills and radioactive materials section (holy [crap]) showed up at my friend's house around 10am and spent a few hours taking readings, swabbing samples and asking questions and after all of that, found nothing but natural trace amounts of radon. The "lid" they think was from the late 1950's and was unrelated to the "vault".