I work at an office typing numbers into a computer. It gets quite boring sometimes, so I go on Netflix while using their WiFi on my phone.
Can they tell what apps I'm using, based on network traffic?
Short answer is yes. If there is any logging on their WiFi router they might not be able to see exact apps, but they'll be able to see the server domain/hostnames that you're connecting to.
You can also look at these question and answers:
Secure WiFi only encrypts traffic up to the Access Point. It's decrypted, and the traffic can be monitored. If it's an enterprise router it's more than capable of logging specific types of traffic. If your work has a firewall in their network then it's even more likely that your employer has the capability to monitor traffic. These do have to be configured, and your employer has to care, but it's very possible.
Domain Name Server (DNS) requests contain the domain hostname that you're trying to reach, and these requests are sent before an SSL/TLS channel can be established and secure.
If you're using SSL/TLS it's possible to see the hostname of the server that you're connecting to in two ways. First is the server's certificate. The common name generally needs to match the domain name that you're browsing to.
Can they link this back to you? This really depends on a lot of factors. If they're really adamant they'll be able to track down the MAC address of your device, IMEI, and other mobile phone identifiers. This is a lot of work, and in the end still might not link you to the traffic.
In all honesty, if they care it's just easier to block the traffic than it is to try and track you down.
Yes they can see the traffic, but maybe not the specific apps you are using. Just the traffic. Depending on the setup, they may have already tracked this. Sometimes they will not be able to tell who did it, but sometimes they will also be tracking the device's name (e.g. "Phil's iPad") depending on certain things.
So while it may be true that a company largely will not know if it's you even if they see the traffic, and they may not know specifically that you are using a particular app to generate the traffic, the fact is they COULD know you are the person generating the traffic to and from netflix servers, if your device's name is a dead giveaway or if anyone kind of already knows your device's name. (They could also know if there is anyone in your office who you have chatted with about this issue).
If you haven't been approached yet in any way, or if use of the network for Netflix or large downloads hasn't come up in some way you know about, chances are pretty good that they don't currently care. However you may also want to not risk it in the future, and instead use your own data plan for things like that.
Yes they can.
(As a fighter of full disclosure I won't disclose here how they can. In full agreement with myself :), since in this case I won't give a key advantage to the bad against the good.)
In most law context it is lawful for your employer to investigate this abuse of its company resources: you are here using resources provided and maintained to permit work of all employees, your colleagues. The key exception to this right to stop a resources abuse is the case were resources are explicitly provided as a convenience for private use. For example if your wireless network is named "Rest area" or "Recreation area".