Mystery solved. In short, it was static electricity build-up in the pickguard. When I was practicing last night, the static and hum started again. At some point, I inadvertently touched the pickguard while I wasn't playing, and the static came through the speaker. So I touched it again in a different spot and got static again. I started rubbing the pickguard, and the static would occur and then stop, so I just rubbed the entire pickguard with my fingers, pressing down a little firmly, and both the static and hum went away. I've never heard of anything like that, so I did a Google search for "static electricity pickguard" and found several videos where others had experienced the same thing. Their fix was using a dryer sheet like you mentioned @Eracer_Team-DougH, except it worked for them, and they demonstrated in the video. They mentioned the cause was playing in a carpeted room in sock-feet. I usually have socks and house shoes on, so maybe I've been getting the same effect. When I had played earlier in the day and thought blowing out the output jack may have been the cure, I was about to go out on an errand, so I had shoes on. No static that time. That would also explain why I couldn't reproduce the issue at the store. So, it looks like the short-term fix is to rub the pickguard, with or without a dryer sheet, whenever static electric builds up. The long-term fix would be to add some shielding and/or better grounding under the pickguard.