MacBook uploading data over the Internet on Wi-Fi connection I can see upload traffic to the internet being generated by a MacBook once it connects to our LAN. The owner states he's not using iCloud and doesn't know why this occurs.

I am not a Mac expert but I may give it go to fix the issue.

What should I check for, apart from viruses? Is there a well known functionality in Macs once they connect to the Internet?
Activity Monitor
To see the number of sent packages you can use &quot;Activity Monitor&quot;.
Open it and choose the &quot;Network&quot; tab. Click the &quot;Sent Bytes&quot; column to sort all applications by data sent.

Little Snitch
You can use a firewall software like &quot;Little Snitch&quot; to monitor any incoming and outgoing network connections. It can display a detailed traffic history which you can filter by application.
It is a paid software but you can use the trial for a few days and remove it afterwards.
Keep in mind that this requires you to install the software on the client's machine.