When a stranger finds a new card, there is no way to activate it, because even if the phone is activated, you have to check your personal information.

However, your personal information is on the magnetic stripe of the card. If it is discovered by criminals, it may be used to make fake cards, so there are still some risks.

So I suggest that you'd better report the loss of your card, so that you don't have to bear any risks in the card.