Albus was unsurprised when the emergency owl arrived from the ICW to tell him that Grindelwald had escaped. His own alarm system had already alerted him several hours before but he'd seen no point in responding, his former friend wouldn't have bothered to set off the alarms if he didn't have a firm plan to escape before anyone could respond.
He attended the summons anyway, curious to at least figure out how the other wizard had done it. He apparated to a safe distance from the fortress, not knowing what to expect and whether it would be safe to come close.
The fortress itself seemed mostly untouched, aside from the twisted metal remains that scattered the ground around it. A muggle aeroplane at a guess, from the fuelage and luggage that was scattered about. The site was crawling with ICW wizards, some herding up the confused muggle survivors that appeared to have already been obliviated by the magical backlash. Others took photographs of the wreckage but it was the Assistant Mugwump who caught Albus' attention with his frantic waving. Albus picked his way up the mountainside towards the Frenchman, who began speaking as soon as Albus was within hearing distance.
'…impact was almost perfectly through the wardstone.' Albus tuned out the babble as he carefully climbed up to the wardroom. The nose of the plane had been melted by the magical backlash as it plowed straight through the heavy stone that had acted as an anchor for all the wards that were active on Nurmengard. He could easily see the daylight through the three solid stone walls that the massive aircraft had decimated.
'Sir?' Albus looked down at his assistant with a measure of surprise, having almost forgotten that the man was there.
'Pardon?'
'He's left a message, Sir.' The assistant repeated and Albus grimaced, hoping that his enemy hadn't implicated him. 'In his cell.'
Albus swept up the now familiar staircase, noticing as he did that there was a set of bare footed prints going downwards, almost lost in the booted tracks of the investigating wizards. He wondered if anyone had noticed them, or worse noticed his own multitude of tracks. He'd have to burn his incriminating pairs of shoes as soon as he returned to Hogwarts.
The cell looked like a wildfire had swept through, carbon stains streaked up the walls and the two pieces of furniture that remained were burned husks. There was a twisted lump of metal on the floor and several blackened pieces of china. What quickly caught his attention though was the message painted in charcoal across the wall.
"I will guard it as promised but I will not be imprisoned." Underneath that was the symbol of the hallows, except the circle had been drawn in a reddish brown substance that was almost certainly blood. It was a message that was blatantly obvious to Albus and his mind was running over the possibilities even as he denied any understanding to the ICW investigator.
He returned to Hogwarts in a state of shock, deciding in a moment to cancel exams so that he would have more free time to search for his missing dark wizard. The vanquishing of the beast in the chamber of secrets would serve as reasoning enough for the rest of the faculty.
It was nothing but a foolish mistake, he realized, an oversight that had been so obvious that it had never occurred to him to correct it. Why stop non-magical objects after all? Anything that could be thrown would have little effect on the castle and it would just cause problems with the weather.
And of course, he had only altered the wards to prevent Gellert from passing through them to escape. They would never have been designed to stop a spell passing through from the inside – again why would Albus have added such a precaution when the imprisoned wizard was wandless. He had evidently underestimated just how powerful wandless magic could become if hones far enough. Again, a foolish mistake when the ancient warlocks had never used wands but had achieved impressive feats of magic none the less.
He pulled out a parchment and began a list of all Gellert's known hiding spots, cross referencing with his schedule to decide when to visit each old haunt. The chances of the dark wizard being in any of these places was small but at least Albus could claim to be doing something. On a second piece of paper, Albus began drafting an official statement to be made by the ICW for when the prophet inevitably discovered this disaster.
He was still drafting this statement when he received another emergency communication, this time from the British Ministry of Magic. Sirius Black had escaped from Azkaban and the only information they had was that he'd been mumbling about Hogwarts in his sleep.
For a minute Albus just stared blankly at the letter, unable to believe this wasn't related but knowing there was no way the two incidents could be. Gellert didn't even know Sirius Black existed and as far as Albus was aware had no motivations to free him. Unless Sirius black had been the one to free Grindelwald, if someone had used a wand outside the wards to direct the plane it would have been far more plausible.
He tried to place himself in the position of Sirius Black for a moment, imagining that he had just escaped Azkaban and discovered himself alone and without friends. It would certainly make Grindelwald an attractive target, especially if Black believed the former dark wizard could somehow help his master. Of course, Grindelwald had the stone, which would mean it was perfectly within his power to resurrect the other dark lord.
Of course, the message didn't fit with that theory but it would certainly make how he had escaped more plausible that alternative of wandlessly summoning the plane and managing to so accurately hit the ward stone. It was easy to plant a message after all, it could have been done to distract him.
So if Black had Grindelwald's knowledge and the stone, it was only a matter of time before he raised his previous master, or Grindelwald seized control and made a move. That made finding Black his highest priority.