Mystery washer

I found this on my drive today and don't recognise it. I don't think it comes from anything I own but a mate came round the other day on his 1150GSA and parked roughly where I found it. Looks like it has a fluid retention rubber inner. His bike was all working fine and he'd just popped in from getting an MOT. Just asking here first before I alarm him

It’s a central heating boiler part.

Thanks a lot Nin. That’s the mystery solved. I had a boiler service in the week and the engineer must have dropped it. I’m used to finding washers and circlips in the garage but not on the drive