Taxes for a non-US citizen present in the US

[AbZurrrd]

I have three important questions that are really bothering me and would really appreciate some help:

1 - I am an international graduate student here in the US, and therefore I am paid by the University and pay taxes etc...everything is clear except for poker. How can the IRS suspect that I am winning/losing money playing online poker? Do they have any way of knowing? What's the risk for me if I don't report it(considering that I'm not a citizen and that poker is not my real source of income)?

2 - On the other hand, where SHOULD I pay taxes? In the US or back home (treaty country in Western Europe)? it's pretty confusing...my understanding is that placing wagers from the US implies paying US taxes...but at the same time, I also play when I'm home...

3 - [that one is easy] which amount is considered a threshold in terms of poker earnings (by the IRS)?

Thanks a lot for any help!!!