Is it possible that an underlying heart problem go undetected by an ekg/holter monitor?

Depends: Usually a physician orders a holter if you have palpitations. If you had palpitations during the holter recording and no abnormality is noted then that is a good thing. If your palpitations occur infrequently you might need a longer recording period using an event monitor. There may be structural heart problems or problems with coronary arteries which may not be evaluated by ekg or holter.