What explanation can you give to a network administrator for why your work computer appears to be connecting to a VPN?

I am assuming that your reasons are reasonable, and not like you're copying lots of their documents off their network. Something like you're shopping, or whatever. Your answer should be something like, "Yes, I'm connecting to resources back home through a VPN." I have no idea what your work's policy is on proper use of their network, but it probably includes that you need to be forthright with what you're doing. If I were the CSO there, I might defend you in doing it, but I would insist you answer questions forthrightly when asked. You could try chatting them up -- "hey, what do *you* think is best? SSH? OpenVPN? I'm using <insert thing here>." In some ways, just assuming that every competent geek is VPNing home is your best strategy. Now there are plenty of workplaces where this would definitely not be acceptable use. Finance, defense, etc. But in just an office, they might (probably?) just want to know that you're not part of a botnet. If they tell you to stop, stop. Get a cellular modem, and put it on something that is *your* device (or get a device that has one built in, like one of the new iPads), not theirs. They can can you for putting it in on their device. Oh, also, read the acceptable use policy and make sure you follow it. Most likely, the It department will warm to a fellow geek and they'll actually help you. But if you make their life hard, they won't be appreciative, and that document says what the rules are. If they have stupid rules, like no Facebook from work, using a VPN isn't a good defense. Find another job, one with people who are reasonable.

You should realize that they are being nice and speak to them accordingly.  They don't have to be nice, but they are. The other side of this is that policy is usually set that circumventing network controls (and that's likely what you are considered to be doing on employer networks and resources) can put assets and information at risk.  They usually have good reasons for the controls and monitoring they have in place and HR penalties are often serious, especially if your habits cause an incident of some sort. What I do when I'm going to utilize my privately owned assets in an office environment is simply that my data never touches my employers networks or hardware; I bring my own laptop and datacard/tether. As I'm often one of those people charged with enforcement and/or authorship of these policies and controls, it would be kind of stupid for me to be the first in line to circumvent my own efforts.  Why should anyone else care if I'm breaking my own rules? Most people figured this out already which is why mobile usage, much of which to social networks, is has grown hugely (and have influenced BYOD in corporateland), and is also a factor in why mobile devices are such a mess at present for data exfiltration. Bonus: Some infographics on present mobile adoption: http://blog.nielsen.com/nielsenwire/online_mobile/40-percent-of-u-s-mobile-users-own-smartphones-40-percent-are-android/

Tell him you don't know what he is talking about. Tell him that it's probably some other machine on the network that "appears" to be doing stuff. From the sounds of it, he'll believe you.