Dear Mr XXXXXX [Since the first sentence refers to a letter received, the name of the person is known and should be used].
Thank you for your letter of 20 May. We are delighted that you and your family had very pleasant [had very pleasant what?] when you stayed in our hotel.
We are sorry to inform you that we are unable to find the pair of brown trousers in the wardrobe of your room. We have also checked with the room service team. We will inform you if we find them.
we look forward to waiting your reservation again. We hope that you will stay with us again.
( this is for formal letter and below is informal)<#
Thanks for your letter. We enjoyed to heard are pleased that you guys were happy at our hotel too.staying in our hotel.
I am sorry, but I looked everywhere for your trousers and I couldn't find them. Have you looked at your suitcases and in the car? Perhaps you misplaced them somewhere else.
Sorry I can't help any more but I will let you know if we find them.
See you soon