UPDATE: SOLVED!! Thank you for all the suggestions! Changed my password and will regularly check whos connected moving forward.
My sibling was projecting Netflix on our tv thru hdmi as the tv isn't verified(?) by Netflix when the screen suddenly changed into some porn. We immediately turned it off. We're the only ones at home so no one in the family casted it for sure. I feel like our neighbor accidentally casted it but how?! Can we somehow restrict that? Help please, I don't want that happening when some visitors are here.
You could setup a dedicated guest WiFi that only has Internet access but no access to your TV/NAS etc.