buck converter, no load start
The MOSFET has a recognized leakage current from source to drain - this gives you an output voltage under no load conditions. To avoid this becoming excessive under light-load conditions, you should have a resistor across the output to sap this current to ground but there will still be a small voltage due to ohms law.

The data sheet for the IRF530 states that the leakage current is 25 uA at ambient with 100V across drain and source but I'd still expect it to be in this ballpark with a few volts across it.