I just got my 2002 Volvo s40 back from the shop and I noticed that there were tiny rust spots on it in
nondescript places. I could ignore them and they would just get larger (I think), but I would rather fix them
and not have to worry about them anymore

What is the best and the least expensive way to do this?

Respectfully

Newport_j

 

really depends where on the car - you should be able to go to an automotive paint supplier shop and get some paint-on rust inhibitor. There may even be a clear option...