One outlet in house is hot while turning all breakers off
Definitely dangerous. The main breaker is for a very high amperage (like 100 amps). If there's no other breaker between the main breaker and the outlet, a high amperage would be needed to trip it. 

I doubt the wires to the outlet could be sized appropriatedly. And I doubt it is allowed to have a regular outlet on a high amperage breaker.

Maybe a sub-panel or a breaker you missed ?