Nice theory, yes. Now back to reality. In the OPs example, Honest Finder goes to police and admits by handing over the suitcase to being in possession of illegal stuff.
If, as it’s likely, he’s arrested and charged with possession, his lawyer has to prove that he didn’t possess it for illegal purposes, but only temporarily in order to bring it to the police. The DA will probably counter with “Does Honest Finder think we are morons? If he had nothing to do with it, he would have left it lying where it was and called the cops (esp. today in the time of cell phones). The whole story about finding it is just bullshit”. So in reality, Honest Finder would have to prove that his story - his innocence - is true, and not the DAs take on the events.
Advice: avoid finding illegal stuff, avoid touching it, don’t carry it, or you are in trouble.
Forgive me but the only valid answers to the OP’s question lie in criminal jurisprudence. For a crime to take place there needs to be mens rea = guilty mind, and actus reas = guilty act.
An “innocent” citizen in the circumstances described can never be guilty at law because they lack the elements required to establish a crime.
In the real world many many different outcomes are possible including a sniper intervening from the local book depository but such considerations obscure the pure question. The answer is No.
This isn’t always the case. Some crimes are strict liability crimes. You can be convicted of such a crime even though you had no intent to commit a crime.
Because the question wasn’t “Am I guilty at law” but “Can they [cops] detain me or charge me?”.
And the answer to that is obviously, and likely, “Yes”. Cops can arrest people, and people can be charged, with a lot. The trial is than for finding out whether the accused committed a crime or not - which partly depends on Honest Finder being able to satisfactorily prove his “mens rea” was not criminal, but honest, while the DA will try to prove a criminal mens rea, both trying to convince the judge/ jury that honest finders still exist / that people are dumb enough to be honest instead of pocketing stuff/ that this is a bullshit tale to cover up real criminal possession.
So it seems that the Model Penal Code gives you a grace period to dispose of any contraband you happen upon. It seems to me that if you find illegal and dangerous materials, the reasonable way to terminate your possession would be to turn them in to the authorities.
Well then, to avoid these problems, the correct action would be to drop the item once you realize it is contraband, then call the cops to pick it up (from a borrowed cell phone a couple of blocks away).
Of course, as a fifty-year-old short-haired middle class white male veteran, I have an unrealistically benign attitude towards the police and would never expect them to abuse their authority on my ass.