Possibly off a bike light?

Must be a cover/blanking plate for a charging port or something...?

Looks like it fits over frame for when adding a pump to cage

I’ve got very similar bits knocking around - mine are from various polar cycle computer/HRM and are basically anti-slip patches for when I fit the cadence sensor, handlebar mount, speed sensor. Could it be this?

Mark II Granada?

Have you got di2? Looks like the bit that sits in the back of the junction a box when it bands onto the stem. If not then it looks like the backing for a light or bike computer

Is it anything to do with where the gear cable goes underneath the bottom bracket - asymmetric due to the cable for the front mech?

Looks like the rubber mount for a sensor that goes on a fork, came with wired cateye computers.

Spacer for a front mech in my opinion

Thin Joebristol is right. I've fitted Di2 3 times now and every time that dam bit of rubber is the bit that annoys me the most as the thing slips out when trying to pull O rings around a stem.

Thanks all

I do have a Di2 - will look at that but as long as it's not a requirement for waterproofing then it can foff
(don't think it's a spacer - is very soft & flexy)