any of the larger crocus will naturalise well. I love Pickwick [purple stripy], Joan of Arc [white], Ruby Giant [rich purple] and Golden Yellow.

I have some of the smaller ones but they get 'lost' in the grass when it grows fast in a mild spring.

I find any of them do well in borders and pots as the grass doesn't compete with them.

I think more likely a squirrel or two have been “planting” or rather transplanting them into your lawn.

or seed from the others have finally grown big enough to flower.

I remember Ruby Giant from years ago. Still going? Wow

I have crocus clumps that I planted in Feb 96 and they come up every year. sometimes a few get dug up by mistake and end up in a smaller clump away from the main one. but I love them. I also have a range of others that are in terracotta pots and they don't get moved and they are 5 yrs old plus, blue pearl and vanguard.