For an instances I have been to a resort and during my check out I see that a bill has been presented and I realize that they have missed out on adding couple of more items I consumed say worth $100 is it obligatory on me to remind them ? or can ّ let go the same as they have missed due to what ever reason from their calculation.

Is it okay to not remind them and move on or is it obligatory to remind and pay or it is recommended to pay.?

Yes, it is obligatory. You must remind them if they mistakenly did not charge you. Often they will appreciate your honesty and will let it go.