As per this question, Persona Non Grata process, in theory, applies to the Embassy, meaning that theoretically, a PNGed person isn't supposed to go to their embassy and hide and everything's OK.
However, what happens in practice? Let's say the sending country and the PNG person decided not to follow the rules. The PNGed person, instead of leaving the host country, simply stays inside the Embassy and refuses to leave it, ever.
This means that the host country can't arrest and deport him outside the Embassy (he never leaves); and they can't arrest and deport him inside the embassy (As per Vienna Convention).
Does that mean that, absent Monty Pythonesque strongly worded protestations, there's no practical (especially legal) consequence to the sending country? Or can such action be seen as a formal violation of Vienna convention (or even casus belli)?