Reasonable isn't the same thing as factual. Two people can look at the same facts and reasonably come to different conclusions. In the school debates I did, you were given a topic and told whether you were the opposition or the proposition. If you were good enough, you should be able to win the debate from either side. Reasonably.
One thing that grates people working in groups is expense management. Covering of 'reasonable' expenses over and above whatever you get paid.
One way of viewing this is that you should spend as you would for yourself. Don't spend more because 'someone else is paying'. Don't order Dom Perignon and Lobster because you won't be the one paying. Be reasonable. But for some people isn't that reasonable? If you come home each night to silver service meals, your reasonable will be different from someone who makes their meal themselves from minimum wage. Some people may be at a dinner who normally wouldn't eat out if they were paying themselves. They wouldn't be able to justify it.
Another way to think of it is 'Think like an Owner'. That makes my blood boil. I always think of Thomas Nagel's 'What is it like to be a bat?'. We aren't bats. If you aren't an owner, you can't think like an owner. If the owner spends less, they get more profit. 'Think like an Owner' basically says put the businesses interests first. The idea being that if you regularly do that, you are clearly someone worth promoting, and therefore more likely to be the boss one day. I don't buy that. If you want to make someone think like an owner, make them an owner.
A third approach is to act 'Beyond Reproach'. Never spend anything where there could be some doubt. Where you could be questioned. The only way to do this is to strip things back to basics. This is the opposite response to the 'someone else is paying' idea. It is other people's money, so even though you are being reimbursed for expenses, spend as little as possible. Avoid conspicuous consumption. We would love it if public servants did this. There are some politicians who donate their entire salary to charity and cover their own expenses. Clearly those politicians are privileged enough to do that, but it does leave a better feeling than paying for the trough to be refilled.
I don't have the answer for this. I do think when we criticise public servants for corruption, we should think how endemic it is in business and social settings. When people feel part of their compensation is in the perks. The 'free stuff'. When there is an open bar at a party, there are more likely to be lots of half drunk drinks. When there is lots of wealth in a country, there are more people who are obese.
If there is some sort of real relationship, I think there is more chance of people 'Thinking like Custodians'. For that to happen, there needs to be wider ownership. Not just of the money left over once the party is over, but of the next party, the next meal and the next time you engage with people.