When it came to tracking and observing, Barty had a finger in more pots than anyone else, and he took the initiative on them, not trusting them to investigate any potential lead as well as he would. And really, he disliked missed opportunities. So perhaps it was nothing, but perhaps it was not. Someone had gone into the library without permission. Apparating nearby, Barty made his way in via a back door. He was allowed in - MLE head after all - but he didn't want to announce his presence. Whether a prank, an overeager nerd, or an outsider, Barty preferred the advantage.
Disillusioning himself silently, Barty moved silently. Someone was there, so he didn't bother using a spell to reveal them - that gave up his advantage. No, finding them went the old fashioned way, investigation. He began making his way through the library, looking for anything to give away someone's presence.