What do you call a statement **P** which has never been formally proved but all the observations so far verify it?

This question comes from this scenario: some people use a software for work. They often use a specific function. When this function was designed, the developers feared that some errors might occur. However, these errors never occur and workers have used it for years. Of course this function works correctly, because it has never failed. However, no developer has ever taken the time to understand how.

In this example, **P** is the statement "*this function is error free*".