Sudden breaking glass

This one may be perfectly explainable. I was in the Den with the younger dog watching TV. My wife was at the computer in the library. The Library opens into the Den on the north side and the doorway was open between the two rooms. Our older dog was in the rocking chair in the library against the eastern wall. We have an air conditioner in the library which is against the eastern wall just in back of the rocker. All of a sudden we get a loud crash of glass. My wife turns toward the noise and sees the gray dog get down from the rocker and run to her; obviously scared. I came in out of the Den to ask what happened? My wife replied she didn't know but it sounded like someone had thrown a rock through the library window (the library windows are very narrow and tall. There are two of them roughly 9 ft. apart.) I checked out the library windows and they were still intact. I even went outside and around to those windows but everything was ok. I came back into the Library quite perplexed because I ,too, heard the crashing glass. Then, looking away from the window and toward our glassed in bookcase I found the cause of the crash, but no reason for it. On the south wall of the Library are 6 built-in book shelves that run from the floor to the ceiling. The bottom part (about 30 inches) are actually cupboards for storage with heavy wood doors. The rest of the shelves have glass paned doors. The weather here has been very hot and damp so we have left the doors on the shelves ajar to ward off mold or mildew (Which works pretty good with the air conditioner running). Upon inspecting the doors to the shelves I spotted one with half of the pane of glass missing. I hadn't seen the broken glass before because it was laying on a shelf that runs at just the top of the lower cupboards. The glass had only fallen five or six inches. How in blazes had it made so much noise? Also why had only half of the glass pane fallen? If the pane of glass was loose; wouldn't you have thought the entire pane would have gave away, not just the lower half? I think its rather strange. Anyway I have to get new glass cut to fit. That's a 22 by 12 inch pane. There are 8 of these panes in each door, some of the glass appears to be original (1857) glass but the pane that shattered looks to be modern.

Little black dog reappears.

I was in the bathroom, at the sink, when I again observed a small black dog which I immediately thought was my younger dog, come into the bathroom and trot toward the far wall. I may have blinked, but I didn't see it disappear, but it did. My dog, we call "Shadow" was definitely downstairs at the time. I definitely saw this small, fuzzy, black dog for several seconds. I'm getting old; perhaps it was a hallucination. I'd hate to think so. This is the second such episode. We did observe a small white with brown spots dog when we first moved here and for several years thereafter but was a jack Russell type and hasn't made an appearance in some time. I'm just reporting the episode. I have no answers.