I always assumed any that had spread had been tiny ones that ended up in compost, I didn't realise they could seed that way. I do sometimes get them growing in the compost bins.
I have been eating some free spinach. I have a salad trug by the back door, but it didn't survive the heatwave. Some stuff that went to seed fell down a crack between a small wall and the edge of the patio. While everything else in the garden struggled, this spinach did really well. God knows why, very little soil and no water, but right outside the backdoor and ready to eat. Interestingly the stuff in the trug either seeded too or died back to the root, because that is all sprouting away again. No lettuce, but lots of small spinach and chard leaves.
I do get stuff that grows like 'weeds' all over the place. Often if a crop of something has failed, something else has already started to grow in it's place. Random squashes is another good one. Rather than dig stuff out, I just leave it and let the gaps fill naturally. It does make some random disorganised veg beds, but is the low work option and the plants that just appear have a massive headstart on anything else I might choose to replace the failed crop.