I certainly think a police officer that stumbled across such a thing would be justified in asking follow-up questions like 'hey, what's this about you going to shoot someone?' and then maybe making an arrest based on your replies/demeanor.

'Probable cause' should involve a degree of certainty, because 'possible cause' would be altogether too loose of a standard. It's possible that you're intending to shootme and you just mentioned saulpw to throw other HN users off the scent. Possibilities are only limited by the assessor's imagination.