"The default" doesn't describe how the sentence is written grammatically, but rather how it is used in practice. When Polish person tells you to take a wallet, they most likely mean your wallet. They might not have said that verbatim, but this is what they meant.
If, in some weird scenario when it doesn't matter which wallet is taken, we wished to make that fact clearer, we would likely said „Wzięłaś jakiś portfel?”.
There are a bunch of irregular verbs in Polish and wziąć is one of them. I can't remember if something like that occurs in regular verbs.
Here's a list of them https://pl.wiktionary.org/wiki/Aneks:Język_polski_-_czasowniki_nieregularne