First I shall look at how to prevent this happening again. If you have had one bad experience, or many, losing a specific item, here are some alternatives.
1 Replace with a cheaper or more easily replaced item eg a phone with top up cards so no thief can run up big bills on your phone.
2 Have a very visible item. eg a pink or red phone. Then you cannot assume that it is somebody else's. It is very visible to you and bystanders.
3 Check whenever you leave anywhere, a train seat, a plane seat, a restaurant seat. Check the seat, under the seat, the table, the nearby floor, items left hanging on nearby wall hooks, items pushed out of the way against the wall. If you later lose an item at least you will know where you did not leave it.
4 I take the address card of a restaurant where I have dined. It's useful for repeat bookings. Once granny left her handbag behind a toilet door.
Keep the phone number of the place you visited so you can call back immediately. I am assuming that you think the manager is more honest and more concerned about customers than staff and will look carefully and prevent the item being kept by junior staff or taken by other customers.
Useful if the item left behind could be taken by junior or temporary staff, or other customers.
Leave your phone number just in case the missing item is found later by the cleaners.
Though if you notice at once you might prefer to dash back yourself before anybody else finds it. And the manager might look on the table but not spot it on the next table or floor.
5 Practice the fingertip check as you leave a place. You might recall the old joke about Catholics who make the sign of the cross. The joke goes that they are actually checking hat, wallet ... and so on. Before taking a journey a plane journey or sitting down I count the number of suitcases and bags and umbrellas and coats and hats. I also do this going through security. I put hats in pockets and attach umbrellas through the label on the coat or the sleeve. I put phones not in use inside an inner jacket pocket.
6
Remove items from jacket pockets even when the jacket is on the back of your chair. A passer by could jolt out something such as your car keys. Or take your sunglasses, door keys or wallet.
7
The waiter might suggest moving your coat to a cupboard. Somebody with a similar black coat might take yours accidentally or deliberately.
8
Handbags left on the back of a chair or on the floor can disappear. You could keep your foot through the bag handle or your feet on the bag. It's dirty. It would be preferable to keep your bag on a chair beside you within sight and reach and out of the way of passers by. You might tuck your coat around it. If you are sharing a banquette keep your bag on your side away from others, or with the opening towards you and a coat around the bag and secured under you.
9
Take a photo on your mobile phone of any coat or bag which goes into storage so you can show the hotel management or insurance company if the item goes missing. You could photograph the item in the storage room before you leave so they cannot deny ever having it.
Note items in your bag before leaving home. Remove keys or credit cards and membership cards you don't need and don't want to risk losing because of the time and cost of replacing them. You could photograph the contents of the bag. I once had a bag stolen on holiday. I had a hard job proving to police which items in a recovered car had come from my upturned bag. I was amazed to find that in making the police report, which would later be used for insurance, I had forgotten half the items in my bag. The cost of replacing items I forgot to mention, the swimming costume, towel, sunglasses, diary, and make-up was far higher than I imagined.
Prevention is better than cure.
Keep a list of places where you frequently lose items.
Lost glasses - look beside the bed.
Lost keys - look under the car seat.
Lost phone - check table before leaving restaurant.
