Same route?
Its quiet possible if theres glass, thorns etc somewhere on your route its why I switched to Marathon plus tyres when I commuted by bike; it would be more unusual for you to find a road / track that is maintained and regularly swept.
Did you not find anything at all stuck in the tyre on any of the occasions because that does sound a little odd, its possible you have missed something.
Its when the inner tube gets caught between the rim and the tyre when you fit it and would normally leave a bigger hole than a pin hole puncture.
Depends on your tyre size but maybe buy a more puncture resistant tyre such as Marathon plus (you wont be putting them on without a tyre lever) or get one of those puncture protection inserts that you put inside the tyre they do help a bit but not as good as getting Marathon plus tyres.
There's also that slime stuff that you can put in your tyres as well but I've never tried it.
Do you have a decent pump with gauge to check tyre pressure?
