One summer I was working at Marysville, CA. We rented an apartment & there was a wood fenced patio with it. We let my 4 yr. old son play out there unsupervised as there was no outside access. One day my wife noticed a lot of spiderwebs around the edges, not easily seen. They were black widows, a lot of them. Manager had the area sprayed, but the kid wasn't allowed out there any more.
When I was a kid we had a block wall with a concrete cap on it. The cap wasn't well built & sections broke off, but stayed in place, so you could pick up sections of cap. You guessed it, black widows in the wall.
I found that wasp spray kills spiders; regular bug spray doesn't do it.