House warm thermostat off
Yes! It sounds as if you probably have the answer to your question. The boiler is on your side of the house. The heat supply lines run under the floor on your side. Depending on how well insulated the lines and your floors are you're probably benefiting or sweltering in the heat your tenant is calling for when they crank up the thermostat.
If the basement isn't finished it should be fairly easy to determine - but I'm guessing it is.