cat suddenly meowing at the window

My cat has decided recently (as in, within the last couple of days) that something in the vicinity of my front door/front window is extremely upsetting to him and I need to know about it. He'll sit by the front door or in the window next to it and meow insistently at me for twenty or thirty minutes at a time. The noise he's making is his "mom come look at this! You need to fix this!" meow, so whatever it is, he feels like it's something I need to know about. He also will occasionally stand on his back feet on the windowsill and paw at the window, always in the same spot - but there's nothing there, it's just a corner.

I live in a third-floor apartment and he does get to go out on the balcony; he also seems to be moderately scared of being outside in front of my apartment, so I don't think he's begging to go out.

I did put a little plant stand and a chair outside my window a couple of days ago, so part of me wonders if he's just trying to tell me that things are different outside - but why would that worry him so much? Nothing has changed in a way that's threatening.

Anyway, it's gotten to the point now where it's beginning to get annoying. I can't get him to stop by shushing him (which does actually sometimes work!), and I don't want to start punishing him for talking to me, but I'm starting to kind of want to shoot him. Is there anything else I can do to figure out what's bothering him so much?