Can a bitcoin transaction get confirmed unexpectedly? Let's asume I make a transaction with no fee and doesn't get confirmed after a while (let's say 10 days).

Could it be confirmed after that if for some reason some miner keeps a copy? I don't ask if is likely to happen but if it is posible.
Absolutely. There are cases of transactions remaining unconfirmed for several days and then getting confirmed.

If the transaction was ever broadcast and is still valid, it can be included in a block by any miner who wishes to include it.