"I've rarely been asked for proof, but when you do, a company business card or email address is usually sufficient. A UK email of the US company should be fine. If it is the email of your company, not Deloitte, then you dont have proof. That being said, note that if the rate is for employees only, and not contractors, (which Hilton is usually lenient on, Marriott tends to be stricter) you are breaking the terms of the rate which could result in your room being denied, you being re-rated to current full price rates, and/or your account shut down and points forfeited. These things don't happen often, but it is the risk you run. I think my company's rate works for contractors, but only when traveling on business, not leisure."
— wildcat12321
"At my property we are very flexible in what we ask for. ID name badge is the most preferred, but we recognise not everyone has that. If they have a corporate credit card that work gave them , as long as it shows guest name on it and "(company name) corporate" we accept that. Even an email inbox we can accept. You work for deloitte? Show me your email account that says the account name "John.smith@deloitte" and ill accept that. Edit: never completely answered your question. I have no idea what the front desk at this particular property looked for? Maybe it was a company they never heard of? Is your company part of a merger and they had to verify that? I only learned recently that Microsoft owns LinkedIn because of a corporate rate."
— hotelman97