I just went through this with two vintage vehicles (freeze plug landing inside). While others may chime in on this, my experience with FINDING freeze plugs in vehicles is....no harm...no foul, however I could not allow this in my vehicle.
I was VERY surprised at how easily both vehicles referenced above, gave up the hidden freeze plug. The key is orienting it correctly and in my case, just pulling it through with Vice Grips (sorry...locking pliers)...no wait...they WERE authentic Peterson Vice Grips.
Both were deep cup freeze plugs as well.