Site Concentrators are great for limiting bandwidth consumption in your network by having all the workstations on your network pull patching and Managed AV information from your server, but if your site had road warriors (travelling salesman / techs whose computers are regularly working remotely), you need to make accomodations.
The easiest fix is to put a company's road warriors into their own site, with no Site Concentrator specified. This will make sure that these machines always pull patching and Managed AV definitions straight from the internet, as they may not always have a connection to the network's Site Concentrator.
Additionally, the Site installation package might not work for these machines and it may be difficult to manually install the agent yourself due to their mobility. Your best bet to deploy the agents to a road warrior's machine would be the Remote Worker installation: email them the one-click silent installer, and problem solved!