Symptoms (or the lack thereof) are not an indicator of whether or not someone has contracted it.
OP has stated they've left their home twice in the past 5 months, the last time 4 weeks ago and they just tested positive.
The chance of them being an asymptomatic carrier is actually pretty low, otherwise we wouldn't be doing 14 day quarantines.
Is it possible OP is a rare edge case? Sure. But not likely.
I'm thinking this may be true. Dr. John Campbell has a fantastic video[1] that suggests we may be overlooking one possible symptom in otherwise asymptomatic populations: Skin rashes. That doesn't mean, of course, that everyone gets skin rashes with COVID-19, but it appears that it may be an early clinical feature of the disease even in people who are otherwise spared the worst.
The theory is one may find detectable RNA fragments, without the person truly having an active infection (I.e. they’ve already recovered).
There is also just human error with sample handling, contamination, etc.
There are people who have flown on flights with actively infectious cases of Covid. Following hygiene practices, none of the passengers got infected.