Part of my PhD focuses on people who illegally dump rubbish in grey-area locales (places where ownership isn't obvious - empty lots, new housing estates with only a few residents [or none], unpopular parks): it's a surprisingly common activity! I've had to observe these people as part of my fieldwork and they're just normal people driven by the desire to rid themselves of stuff. This could seem completely bizarre so I can understand why this behaviour would raise a lot of questions. Same mentality as littering I suppose!
Just some slightly related speculation... perhaps those who reported the suitcase/bones to police (or came across it without reporting) didn't think anything of it initially because they'd check up on rubbish regularly, expecting to see strange objects (hence the return visit of the initial discoverer). There's an estate near me in VIC which I visit every so often, and part of my walk includes checking out the random stuff dumped there - anything from boring (household garbage, childrens toys, clothes, and smashed CRT tvs) to very, very weird (animal skeletons, bags of medication, even a giant pile of police checks from one of those dodgy TAFE institutes...!). Looking back on my past visits, I can't even recall seeing anything truly suspect or worth calling police over - perhaps others who saw the suitcase are in the same boat, and simply don't remember the suitcase. I can also imagine some people would be reluctant to mention that they'd seen anything lest they face a barrage of questions from police as to what they were doing in the first place - not that this is any legitimate reason not to speak to police! I'm thinking of the illegal scrub campers here.
Just some thoughts which kind of meandered a little bit

hoh: