Most, if not all, disk galaxies have a thin (classical) disk and a thick disk. In most models thick disks are thought to be a necessary consequence of the disk formation and/or evolution of the galaxy. We present the results of a study of the thick disk properties in edge-on galaxies from mid-IR imaging obtained in the Spitzer Survey of Stellar Structure of Galaxies (S4G, PI Kartik Sheth). We fitted one-dimensional luminosity profiles with physically motivated functions - the solutions of two stellar and one gaseous isothermal coupled disks in equilibrium - which are likely to yield more accurate results than the other functions used in previous studies. We found that thick disks are on average more massive than previously reported, mostly due to the selected fitting function. Typically, the thin and the thick disk have similar masses. Our results tend to favor an in situ origin for most of the stars in the thick disk. In addition the thick disk may contain a significant amount of stars coming from satellites accreted after the initial build-up of the galaxy and an extra fraction of stars coming from the secular heating of the thin disk by its own overdensities. Assigning thick disk light to the thin disk component may lead to an underestimate of the overall stellar mass in galaxies, because of different mass to light ratios in the two disk components. On the basis of our new results, we estimate that disk stellar masses are between 10% and 50% higher than previously thought and we suggest that thick disks are a reservoir of "local missing baryons".