if you have had no flats caused by punctures, as stated in your original post, then the problem should be easy enough to determine. put the inflated (about 100PSI should be enough) tire/wheel in the bathtub with about a foot of water in it and see where the air is coming out. if you can't see any air bubbles coming out, then there is no leak or at least nothing to be concerned about. and your flat tires are caused by the normal porosity of latex tubes compared to butyl tubes. IOW latex tubes normally lose enough air after a day or two to be considered by many cyclists, 'flat'. mine do.