on site vs at site

Some examples from the web:

• Only a project manager on site.
• Safety precautions on site are absolutely minimal.
• Spin him around here. I'll process him on site.
• We need docs on site, stat.
• Cooper thinks she's still on site.
• I need CSU on site here at the Carlton Hotel.
• Catherine, notify Grover, have SWAT meet us on site.
• All right, we're on site. Copy.
• Get back to you when we're on site.
• The company have a nursery on site.
• We have vehicles on site for your team.
• Repeat... Simmons is on site with Hydra.
• Keep all S.T.R.I.K.E. personnel on site.
• Jack Dawkins and Devgru can be on site within 20 minutes.
• We have federal agents on site.
• Director Heyworth, the agents are on site.
• Eagle One on site, beginning rooftop search.
• 19-7, we need an ambulance on site.
• But just remember, there are no support staff on site.
• This is Two arriving on site.

Some examples from the web:

• I'm at site 6412-Foxtrot with Teo Braga.
• The plutonium at Site X will never be pure enough.
• Gamma sensor at Site Nine is not responding.
• But, Charlie, we're not using the plutonium at Site X.
• What you saw at Site X was a pilot plant designed to work out the kinks.
• With the plutonium that they are making at Site X,
• You at Site Nine yet?
• As at site number 119, the railway bridge and electricity station at Cloudheath, Suffolk, where Tulse Luper says the information on signs is significant.
• That is why all EMAS registered organisations should report on core indicators at site level, though the regulation does not explicitly require this.
• "at site, bring cash, no cameras."
• Has no one at site abandoned?
• In any case, information on trends in emissions to air and water, water consumption, use of energy and the amount of waste should be provided at site level.
• time course of onset and signs of toxicity, including dermal irritation at site of administration, if any, for each animal;
• The establishment, at site level, of the conservation objectives, as well as of the appropriate conservation measures aiming at maintaining or restoring the habitats and species of Community interest at a favourable conservation status falls under the full responsibility of the national competent authorities.
• We've had static at Site Nine all day.
• I spent three years at Site X.
• Gentlemen, we've just gotten word that more than a gram of plutonium will be shipping imminently from the graphite reactor at Site X.
• Director of Operations at Site X. He says he needs to speak with you immediately.
• There's a mobile link up at Site 26 we can work out of, way up in the mountains.
• The workers at Site X barely know what they're making down there much less what it's used for.