As an ex Pest Controller for many years, don't wear yourself out trying to find how they get in, if it isn't obvious then you will most likely never find out. Just deal with the issue when it arrives, remind your pest controller that it is a part of his job and responsibility to find and remove rodent carcasses. To be honest, getting a smell and flies after the rats die is not that common, especially during the winter. Traps can be effective in some situations, however, rats are really not that easy to trap like mice. I would stick to putting down poisons. Don't delay, dealing with the issue, rats breed very rapidly, a lot easier to deal with a pair of rats than a whole family.