Blew rear freeze plug on 408

I've never seen freeze plugs pop out. If one did I think an installation issue. OR a messed up sealing surface in the block etc. Usually in service, they rust out if steel. I am currently installing tall brass core plugs with Permatex 2. I would pressure test, get under it, pull the lower inspection cover and look up behind flywheel basically be sure it's a core plug. If you can't figure what it is, pull the engine do the head gasket and rear core plugs in one job. Really not that bad pulling the engine, it's psychological. Easier than laying over fenders and doing heads. In our mind we think pulling engine is alot, but it isnt if you have to get at the rear core plugs...