Considering that a lot of land in a city isnt your garden/house (shops, offices, parks), I would suggest that that would tend to balance out the risk of this being a SAZ.
I take your point that this is a data point of one. But if I found one straw in my garden, it is astoundingly unlikely that is the only wild straw in my city. I wouldn't start putting a figure on it from that, but it does suggest the lower bound is much higher than one.
Having never found a straw in my garden my intuition is that (not finding straws in every garden) is more likely than (finding straws in every garden) but obviously I have no valid basis to put a number of any kind on that.
Now while I think the original post is an example of Hasty Generalisation, I think the bigger issue is the generalisation combined with a sample bias. Self reported data points tend to be interesting because otherwise they wouldn't be reported. My thesis is that regularly finding a straw in your garden is interesting because it is unusual.
It's far more likely that I live in a stray straw accumulation zone than straws are uniformly distributed over the entire city's gardens by some method.
[0] https://en.wikipedia.org/wiki/Faulty_generalization#Hasty_ge...