To say that a person missing his/her wallet is a case of sofeik pikuach nefesh is a huge stretch. Even beside the points that Joseph made (which are good points), the chances that this person is going to need emergency medical attention in the near future are very slim.
If you say this is a case of sofeik pikuach nefesh which is docheh shabbos, you can probably use the same logic to justify being mechalel shabbos in any situation that arises.