Rudely interrupted by Other Plans of Sirius' surprise birthday party, Al hadn't had a chance to give Snape's books a thorough cleaning yet. Naturally, he was very distressed regarding the fact that Snape's grimoires had to greet the New Year with two weeks' worth of dust on their spines. However, he planned to remedy the situation as soon as possible.
Al was warned several times lately to be careful, so he was, watching for any suspicious activity, no matter where it might originate.
He Apparated to the Manor, only to be greeted by a Swarm of Shady Security who sent him away the first time, until Al Apparated back home for his ID, and presented it to prove that, yes, he was a Potter, and an Al, and not Polyjuiced, and not Confunded - 'no, honestly, I always talk like this!' - and had a reasonable cause to enter through the gate.
Suspicious... Very much so!
Just as he thought he was done, the Second Swarm of Shady Security stopped him on the way through the door and have asked exactly same questions, eyeing and fondling his ID and wand more than was proper in a friendly greeting.
Perhaps there was something amiss at the Manor. Al began to worry as to what it might be.
Imagine his further shock, reinforced by prior worry, when, as Al peeked into the study, he saw bare shelves, bare walls, and bare desk.
His eyes widened.
His face turned white.
His jaw dropped.
His mind was capable of only one echoed panicked mental cry of: B-but, where'd all the BOOKS go!?