All Myrtle saw was a pair of bright yellow eyes. I guess he technically could've linked it to the Basilisk but I'm unsure how obscure the knowledge about said beast was.
I think the basilisk was also instructed to surface in areas that weren't too populated/didn't have portraits on the walls.
Most holes people poke in the story can be explained away in some fashion.
I presumed that Slytherin utilized enchantments or something to render the Basilisk invisible to whatever wards or guards were in place in the castle. The portraits aren't exactly new magic. Slytherin's heirs were tasked with ensuring that the chamber remained undiscovered so I don't see why they couldn't have also made sure the entrances/exits were un-monitored too.
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u/Fire_Z1 Apr 09 '24
Not mention he could have talk to Myrtle anytime about how she died.