The 520, 550 and 360 all have "vibration" sensors which
are designed to put the remote to sleep after a certain
period of inactivity (20 minutes??). They turn the LCD
off to conserve battery power (not that the LCD uses much
power anyway...)

This is different to the 880 and 890 and 720's tilt sensor,
which is designed to light up the remote when you pick
it up.

If the 520etc had tilt sensors to light up the remote
it would drain the batteries in no time. The 880etc can
do it because of the rechargeable battery.