I understand what u mean, "believe whatever we want to believe".
However, I do think mind is a very powerful tool to create, as we always hear "what you think, what u will become". I would suggest those whoever easily get neg emotions/ thoughts & such perception might give only more unfavorable experience, so I would suggest to try to be more sceptical & not to believe smell from unknown source, to be exactly from invisible strangers, as we should not create unnecessary karma, such as attracting unwanted spirit to start hanging around with you (let's say an angry ex-smoker) as we do believe this being is trying to give a goosebump & give ppl fear, etc. when this soul is in the instable state of afterlife, like can't accept the incident that happened to him & made him lose his life & frustrated.
So even if it's true they are there, they will lose interest, if u ignore them or dont give the attention nor what they want. Law of attraction. The more you believe their existence, the more they are attracted to you & they can't get enough to be around you, as your fear gives what they need.
On the other hand, if you admit such smell just as a one-time experience, for example u need a comfort & confirmation from your deceased loved one to give a sign that he/she's fine & still alive. Such experience gives different impact; it gives a peaceful benefit from both parties, so that human left behind & deceased loved one can move forward to the next level of their own life where the soul knows that their family will do fine without them & they can continue their things & the family know that their loved one is peaceful, so they can continue with their life mission on earth. Don't u think the same as me on this one?