Just to jump on these, yes: There can be proxies configured in one browser, but not in another client, on the same system. Even if they're on the same network, the computers might be configured to use different proxies. Often these are intentional on the system administrator's part (routing traffic through some sort of monitoring firewall, or through a VPN that gives access to corporate resources), but sometimes the user isn't aware of how their administrator configured things. (And sometimes there's malware that configured proxies in a way that really isn't what the user intended.) There are a lot of possibilities, so it's really hard for us random people on the Internet to tell you what your users are seeing.
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