wrote:
Driving home last night I overtook a cyclist with what from about 100m
away I thought was a very dim constant rear light.

I have a cheapo rear LED light which I use in constant mode. One thing
I've noticed is that with freshly charged batteries, it is very bright
and hurts your eyes to look at it directly.

As the battery power dies though, the LEDs stay lit but appear dimmer
and this isn't always obvious as it it with a 'normal' bulb light.
Maybe this guy's batteries were also dying.

peter