It's normal for tires to bleed air slowly. The rate depends on the width and pressure, with narrow high pressure tires being faster than fat mtb tires. Your loss may be at the high end of the normal range. It's possible that you have a pin hole leak, or possibly it's leaking through a loose valve.

If you can live with it, just buy yourself a basic ($10-15) floor pump to keep your tires topped off. If you don't want to live with it you'll still need a pump because, even in the best of circumstances, you'll still need to top tires off every 2-3 weeks.