Severus arched an eyebrow as he read Theo's note. He then did a quick internal inventory and nodded to himself. The Fidelius had been taken down properly rather than any other option. If Rodolphus was already dead the Fidelius would still be there and he would now be one of many Secret Keepers. But it did mean that there was a strong possibility that Rodolphus was dying and had removed the Fidelius to bolster his strength for just a little longer. Severus himself would not believe the man was dead until he had seen the body himself.
Theo,
He is likely still alive though potentially mortally ill. If you have a way of knowing when or if Rabastan returns, I would put that in motion. That may well be our first indication that Rodolphus has died. Rabastan would want to claim the manor as the last remaining Lestrange.