The foul odor is coming from an empty pipe between the utility room and the garage. Since the empty pipe ends under the garage and is presumably not airtight, an animal (probably a mouse or rat) got into the pipe and died in it. Since the other end (the opening to the utility room) was also only conventionally sealed with a layer of construction foam, the smell was able to spread through the expansion joint between the screed and the wall in the utility room.
What do you think? What should be used to re-seal it? The basic idea was to put 30 cm of foam in the pipe and reseal it. Then fill the entire pipe and the cut out area with a sealant to remove the odor. Maybe epoxy resin?