When a creature is invisible but not hidden, one could argue that the players still know it's general location. If a creature becomes invisible and not hidden does it remain on the battle-mat?
Does a simple casting of Detect Magic reveal the presence of magical invisibility?
From the PHB, Appendix A on Conditions: INVISIBLE An invisible creature is impossible to see without the aid of magic or a special sense. For the purpose of hiding, the creature is heavily ...