...to suspect that a fellow lawyer who has just put out a client briefing titled "Am I being unreasonable?" must be a mumsnetter?

[SixSpotBurnet]

I've just noticed that a fellow lawyer has published a client briefing (on the extent to which contracting parties can limit their liability to each other, if you're interested) and called it "Am I being unreasonable?".

Come out come out wherever you are ...

it could be xenia!

Has to be a MNer!

Electric - I was thinking that too.

There are quite a few lawyers on MN

Totally must be.

AIBU doesn't seem the right question here.

I just don't think suspecting someone of being a Mnetter involves being unreasonable.

I think OP was asking if it was unreasonable to conclude, given the evidence, that the lawyer in question was a MNetter.
So, no YANBU!