Short answer: Yes. But there might not be anything to worry about.
VPNs keep your internet activity hidden, but if someone knows what they’re looking for, they can tell when you’re using one. That might sound alarming, but as long as you’re using a reliable VPN and it’s working properly, there probably isn’t anything to worry about. First, let’s explain how someone could tell you’re using a VPN. It all comes down to IP addresses.
When you connect to the internet without a VPN, someone monitoring your activity would see you connecting to multiple IP addresses for every website or online service you’re using. If you’re using a VPN, however, the only IP address you’d be connecting to would be the VPN’s proxy server, which is where your traffic is funneled. Someone could cross-reference that IP address with known VPN server addresses and confirm that’s what you’re connecting to.
So, who can see what IP address you’re connecting to? Your ISP can see your router activity and, depending on where you live, may share it with government agencies—who may have access to your internet traffic in the first place. It’s also possible for a wifi network’s admin to see what address you’re connecting to, and there are ways to sniff out another user’s browsing data if you’re connected to the same network.
However, the only thing they could confirm is whether you’re using a VPN. All the traffic going to the VPN’s server is still hidden, and no one can figure out what you were actually connecting to—as long as you’re using a true no-log VPN, that is.
It’s also possible for VPN connections to fail, which would ostensibly reveal your traffic to anyone keeping tabs, which is why VPN kill switches are a necessary feature.
Again, knowing someone could see you’re using a VPN might sound scary, but fear of being monitored is why people use VPNs in the first place. Issues only arise if you live somewhere where they are illegal. VPNs are legal to use in the U.S. for example, but banned in China. Using a VPN may also go against a shared network’s terms of service, which is a common rule in offices and college libraries where you’re using that organization’s computers.
It’s also possible for a website to recognize a VPN’s IP address. This isn’t normally a problem, but some websites may block VPN servers from accessing their content, especially if a VPN would allow users to get around paywalls or strict region blocks. This is uncommon, however, and many VPNs have contingency plans to work against such restrictions.
So bottom line: People can tell if you’re using a VPN, and most of them are the exact people you’re trying to hide your browsing activity from in the first place. But as long as your VPN works as intended (and is legal to use where you live), they can’t see what you’re doing and you won’t get in trouble.