Lex was not unaware of what was happening in the castle. Even if he ventured into the jungle, and left the castle in Luthor's capable hands, it was no excuse for letting down his guard. He'd made that mistake before, and wouldn't do it again.
Just as he left a spiritual mark on Fenrir and the others, he had left a mark on the castle and many of the workers there as well. When the armies of Abaddon appeared, he was the first one to inform Luthor that they were unlikely to target them. Instead, they were gathering to face the jungle.
Similarly, despite the many inherent dangers of the jungle, Lex sensed that the jungle did not actively reject him, and even treated him as a part of it. He did not know why this was so, most likely because he had a soul, and was not filled with the ever present hunger that belonged to all creatures of Abaddon. Either way, the jungle would most likely end up being a refuge for them if they entered.
