salinity

[TC51]

Hi guys
I have a 33 gal reef has been ruining for approx 7 weeks now, I have been maintaining a salinity level of 1.025 without much problem until now. I did my usual morning water top up and tested my water, to my surprise the salinity level was at 1.029. I am just using the cheap coralife deep six tester.
I have not changed any of my procedures since starting the tank, so I am confused as to what would cause this sudden rise in the salinity level. I am new to the salt game and would appreciate any advise as to what would cause this and what I should do to bring it back on line to were it should be.
Thanks
Tom.