During Fire Inspector Wilbur Ricker's testimony in United States Federal Court his photo of the third gas can was shown as an exhibit.  The small building that housed the oil fired hot water boiler can be seen.
This is a photo of the back door of the cottage before Mike's murder.
The photo above is from Hartford Fire investigator Michael Meagher's file and the photo below is one of Fire Inspector Wilbur Ricker's photos that was also an exhibit. Ricker took the photo the day he found Mike's body. He said the reason he took the photo was because he was not taking the can.
This is a strange place for a gas can with fuel for a oil furnace to be located.