well they cannot detect it of course. i think the terms are more related to sharing your connection by sharing your account details. but i guess that is no longer possible because the account details are locked with your landline number. but in theory, the connection you have is supposed to be used by you. and of course, if your neighbor turns out to be a terrorist, you are going to get into deep trouble.
Is it illegal ? I mean, if you are in bachelor set up, you may take connection in your name and share it with ethernet or WiFi through your friends in other room (in case of a flat) who may be paying directly to the land lord and is no way related to you. Isn't the same here ? I think unless you are making profit out of connection say by offering it as a cafe or something, I think it would be quite legal to share a connection. By that way you can get plans of higher cost and share the bill.
But as admin mentioned, it won't give any practical problems.
haha..happened to an American citizen living in Mumbai few years back. He had an unsecured wifi and the terrorists used it to send an email. Though he was let off eventually I believe he had a tough time in between.