Wouldn't the typical sign (yellow triangle with camera icon, and text "CCTV in operation") do the work? Nobody will read a letter on a door, but a shiny yellow

Wouldn't the typical sign (yellow triangle with camera icon, and text "CCTV in operation") do the work? Nobody will read a letter on a door, but a shiny yellow poster A4/Letter would definitely attract the eyeballs. Or is it a regulatory thingie, that you "must" have the letter? (genuinely asking - are you in the US)?