- Are inconspicuous PIR sensors less of a deterent than a big bell-box?

Internal PIR sensors are no deterrent. By the time Mr Scrote The
Thieving Scumbag is close enough to see your PIRs he's in your property.

The standalone mains-powered PIR sensor has a row of Knight-rider style red
LEDS, which should attract the attention of anyone peering in through a
window. OTOH, they'd look silly and possibly reduce security if the PIR was
installed outside.