raggydolls22 I don't know in terms of attachment and in your shoes I would keep an eye on it, maybe a record diary to see how often it is happening.
I think strangers, as in people we do not yet know, come in many forms. There is the mum you see everyday on the way to school but do not know or speak to, the person serving you in a shop, and the total stranger you have never seen before. Any and all could be a saviour or danger in the right or wrong circumstance.
The thing is appropriate behaviour from the adult and from your child.
Touching or hugging or lifting onto equipment in the park, from strangers, in my book, not OK. Talking, smiling, waving etc from strangers, fine.
So in my book decide what is right for you and your child and follow that through in what you teach them.
I too am an adopter and also a birth parent and I know for children who have been separated from birth parents/foster carers and though the care system there can be additional issues with strangers and concerns which do not apply for kids who have not been through these experiences.
It is is also vital for children to know they do not need to be in close contact or physical contact with adults they do not know. For adult strangers they do not want to talk to, they do not need to talk to them, unless (again) it is appropriate - e.g. talking to a new teacher who they will need to talk to, speaking to a swimming instructor or the life guard at the pool if they are asked a question relating to safety etc. Of course I am thinking well ahead here! For your little one it may be the new nursery teacher.
There is a mistaken belief that forcing kids to talk to people makes them less shy, I don't think it does, it needs to come from within. If your child is naturally outgoing that is great but just watch it in case this is a way for your child to get validation, because really they do not need to get validation from unknown adults.
That's all a bit deep and it may just be they were topping over a bit!