I've been told by a professional that "unrelated" generally means work that might be in a different profession - and that to a court, "computers" could be considered a single profession in this sort of case. Work done in the field of "computers" outside of work, and you work for a "computer" firm? Related. Ergo your employer owns the IP.
So yeah, don't depend on it. At all.
This is the advice I was given when verifying that I had the IP relating to my startup. To avoid any doubt, I got the IP signed over with a contract I paid a solicitor for. This cost money, but more importantly it helped that my former employer was willing to do this.
Thinking about it, this default is the sort of thing that stifles innovation. If the Government wants to promote innovation, perhaps they should change the rules.
Edited to add: I'm in the UK.
Cite, please. Your claim directly contradicts formal advice I have received from multiple employment law specialists in the UK, on several occasions over the past few years, most recently last year.
You're right that it's always better to be clear in the contract anyway, though.
Then it depends what your relation is to the company. Do you want to tell them you have more ambitions than being a loyal employee ? If your bosses are a 'general' management and not a founder etc., I very much doubt you will achieve anything positive yourself. I believe these contracts are a norm here from what I heard as well. (I quit in the end before starting my venture).
But I agree it's better to just get it agreed upfront.
The company owns any IP I produce, and I'm forbidden from working on side projects... without permission from my manager. So given permission (in writing) to start a project, it'd then be all mine.
Agreeing these sorts of things prior to spending time/money on your own stuff should avoid most of these legal haggles.
On the other hand, if the outside work provably used no employer resources, and provably is unrelated, then you can probably defend yourself against an action. But you really, really don't want to depend on that.
Get it sorted up front. Get an explicit agreement with your employer that non-competing work done without their resources and without impacting your work, belongs to you.
Otherwise it's murky and dangerous.