There is no moral thingy because in a job you are getting compensated for what you do (sometime it is less or more than what you deserve). Assume a case where a long term employee gets ill and cannot work - the company will just let them go. So there is just a contract between employee and the company. So it is not wrong to look out.
Consider the case where a manager and team helped you handle some personal issue (may be while undergoing some treatment) by sharing your work load. In that case the employee has moral obligation to the team (not to the company). And should make sure the team's help is reciprocated in some way. So looking out for other job at this point may not be the right thing to do.