Invocation Of The Law Matrix.

The demon lord's dedication to snooping on portal creation helped her to spot Aeternus opening a portal. She picked up quickly on a strangely powerful abyss altar open a portal in the domain of high-rank demons. Such a portal should not be seen among high-rank demons. She collected more data and determined it to be Aeternus but she didn't make any rash moves yet. 

She is familiar with his energy signature so she can say for certain that he opened the portal but she waited to confirm that he actually passed through the portal. It is important that he went through that portal instead of trying to fool her into thinking he went through the portal. 

He could have opened multiple portals to fool her so she decided to use the strangeness of his aura based on his weird energy to confirm his passage out of the abyss. It was only when she was certain that he crossed the portal that she informed her subordinates to prepare for war.