You should have returned it to front desk (wouldn't be fair to the next guest to get a used umbrella if they wanted to purchase it).
I actually thought for a split second, I could place the umbrella back in the vinyl bag with a ziplock bag type of opening and put it back in the minibar ... but thought "nah I got morals."
At SGS in BKK, they have umbrellas in each room for borrowing as well as more at the lobby entrance. And they have these nifty plastic bags at the hotel entrances which slip over the collapsed umbrella when you come in from the rain so it doesn't get everything near it wet.
On a similar theme. This stay in New York was my first stay at a SPG hotel outside of Asia and I have never come across a hotel running out of umbrellas at any of my stays before. If the hotel does run out of umbrellas there's nothing really I can say or do, but there's an extra umbrella for sale in every room in the hotel. I've gotten used to hotels having loaner umbrellas in the closet and if not, the lobby always providing one.
Question is, is the umbella from the mini bar the same as those they lent out for free, if not then the guest should be told to get one from the FD or wherever, if its the same one then they should be told to use it but when you return make sure its noted by the person its returned to
Unfortunately, I can't say because I never got to see the loaner umbrellas.