As long as you don't lower the temperature she won't go into a proper cold brumation, she may slow down and be less active though. They know not to brumate with food inside, which is why they often go off their food at this time of year, however lowering the temperature would force her to brumate, which would be a bad thing to do after she's eaten recently.
They like odd corners and places. And remember garters are semi-aquatic, so spending lots of time in a water bowl is quite normal. If you think she's due to shed keep the humidity above 50%, it's a good idea to put moist moss in the cold hide.