Let's go through some options in my usual fashion and fail to come to a satisfactory conclusion.
1) They went out for the night as planned, got drunk, got lucky, got waylaid and stayed out all night.
Problem: they left the key and took the contents of the case. So surely had no intention of staying over.
2) They had plans overnight but, perhaps just arrived from the airport, they needed somewhere to stay for several hours and a place to store his luggage.
Problem: They took whatever was in the suitcase. Therefore they didn't seemingly need overnight storage. If it was just clothes etc, perhaps to change into, why so heavy?
3) Drug dealers. It was 60 kilos of something from miami vice. That would explain very little. If the suitcase was necessary to hide the contents, why and how did they take the contents out of the house without it? And if the suitcase was unnecessary for transportation, why have it in the first place? An no, its not really big enough for a body or the like. It wasn't growling like a tiger either. Essentially he just brought it over the doorstep, commented oh when he saw he'd have to carry it upstairs..i said well you can just leave it in the downstairs room if you like. He said no they need it to change their clothes etc. He commented as he got to the top of the stairs that god it was heavy..it weighs 61 kilos he said.."I'm strong but this is heavy for me!" he said cheerfully.
Problem..if it was just clothes, even boots, books and shoes for added weight..why and how did they transport it from the house without the case?
4) My favourite, after vampires, Russian sleeper agents. Perhaps there was weaponry in the bag. Problem: see above.
Final guess...some kind of gift, to be wrapped and and delivered to someone last night. Suitcase would be unsuitable and not needing it any more they said F it, just leave it here.
Maybe i should make this a poll.
Edit: final clue for aspiring detectives. One last look at the suitcase shows it has two airport luggage tags on the handle. One of them is old, from August. The other has no date, but is an orange label declaring it to be "heavy" and is filled in as "28kg". Whether that was an accurate weight from yesterday or from a past trip, who knows.. but in either case would suggest very heavy weight in it has been transported through Airport security at least once. A further strike against villainy.