I’d say generally speaking it’s wrong but there are some circumstances under which it might be justifiable like if they are about to rip you off and take all your savings and you need some evidence. Let’s say they already withdrew large sum of money and refuse to provide explanation. Or you are afraid to ask them because they’d find other sneaky way to take your money. But even then I think it’s better to hire private investigator and let them look for evidence. Plus I am not sure if tracking app would be admissible in court. Is there a reason you are asking?