return imac studio monitor with no box, just 5 days ago

You will need to check with and return it to the same Apple Store.

Note that Apple does have a 14 days no questions asked return policy.

Sales Policies - Apple

I never toss my Apple boxes on a whim that I might later return, move or sell.

glducci wrote:

I purchased the monitor 5 days ago, and i just threw the box out. Will apple accept my return. Bought in person at an apple store. thanks

The short answer is no.

From the posted link…

For any undamaged product, simply return it with its included accessories and packaging along with the original receipt (or gift receipt) within 14 days of the date you receive the product, and we’ll exchange it or offer a refund based upon the original payment method.