Not all Poly heads have casting plugs....= )
Unless you are re-building the heads, or engine, I won't worry about them, they are small (if present)and rarely leak. The same goes for the ones at the rear of the block. Check them out the next time the engine is pulled, otherwise, unless you see a rust line coming from the bell housing and have overheating issues and see steam coming out of the flywheel...leave them alone.