I don't know either. For me it is usually confirmation bias stuff, like if someone is looking for signs of mental illness in me that they saw on my medical records, they will use any evidence or even just normal behaviors and exagerate it to fit their suspicions.
I got called anti-social, unempatetic, and paranoid/out of touch for arguing that the ink blot test was stupid, and saying that one of the cards looked like a spider or crab and the other a bat lol.
(just for shits and giggles. I was just sleep deprived and didn't want to deal with some, in my opinion, pseudoscience so I just made random shit up to see what they write down. And they still managed to actually come up with semething)
Otherwise, unless I really fuck up and say something stupid like, ''I will eat your soul and set a innocent orphanage on fire (sarcasm).'' as a answer to what year is it, most people don't suspect a thing.
Most people are actually oblivious when it comes to seeing signs of mental illnesses, it's one reason why ctb feels so sudden or out of nowhere. Even psychchiatrists can't correctly diangnose people 100% of the time, so a average joe is going to know next to nothing.
When most people think of mental illness, they think of the most severe, and sadly associate mental illnesses with violent behavior. Which is really sad when people reach out for help and aren't believed because they don't fit the 'crazy person' sterotype so it is assumed they are faking or lazy.
If they don't know your medical history, it really is a slim chance people notice anything. They are more likely to assume you are drunk or high, or even lazy than think you have mental illnesses.