This means you are unable to prove your identity to Polish police when requested to do so and failing to be able to do that means you can be locked up until your identity (and immigration status) can be established (which is impossible given that you have no passport).
So, what does a person do if they don't carry their passport everywhere or it's with the UK Govt for renewal etc? I spoke to some police here yesterday about an unrelated issue, had they have found me 'a bit shifty-like' and wanted my ID, how would I prove it to them when all I'm carrying is a credit card and bus pass?
Being forced to carry ID is not something I'm comfortable with, frankly, for lots of reasons.
I wonder if Constable Polski would have accompanied me to the flat to retrieve my passport?
But if it were in Dusseldorf or lost with a courier, then what?