This is anecdotal but I think heroin, or any drug, addicts often seem normal to those who have never taken drugs or been addicted. I remember my step brother mentioning that my cousin, now dead from an overdose, was high once when he seemed perfectly normal to me.
Once you know the signs, either because you’ve done drugs or been around people who have done a lot of drugs, you see that a lot of people everywhere are high on something.
People always think all opioid addicts are slumped over passed out all the time. And that is usually only when their addiction is really far gone. Early on heroin addicts take moderate doses and keep on functioning. They watch a movie or play video games. You hear people talk about how “clean” the high is, because unlike alcohol they wake up the next day feeling great with no hangover. And they aren’t physically dependent at first, so they think, hey, this is nothing like all of the scary PSAs said it was.
At a certain point one is "maintaining 0". High is "not on the floor writhing or worse."
That said, there can be signs. You may find them normalized from exposure. Or, perhaps: hidden, your cousin maintained appearances. With time and circumstance, everyone slips. Sometimes it's seen.
My (much older) brother managed his addiction, and appearances, well at work for decades. Now clean, thank goodness.
I have my own addictions, and to be honest, I expect everyone to notice and no one ever says anything. Sometimes I wonder if it’s that they don’t notice, or everyone is just too polite, or a mixture of both.
It’s not necessarily that you’ve done it. It’s that you’ve spent time around addicts. Often these are overlapping life circumstances though..