This is a tough one. As a preschool teacher, I would say that being consistent in your rules is important. However, the Montessori teaching method (and many other early childhood thoeries) would say that although you can often train young children to share, they are not yet able to understand why. On the other hand, I think that some children may not be ready to understand personal property at that age and may not know that it is not a communal toy- I particularly notice this in children that attend daycare that are used to sharing toys in communal settings all day long. Young children that do not attend daycare tend to be far more attached to their toys.
A few months at that age can mean major differences in their development. It has also been my experience that many children are terribly shy and just don't have it in them to ask anyone including their own parents.
As you consider it a courtesy to ask to use them-as I do, I don't think it would be right to say anything to another child or their parent- because it would be like telling another child to use their manners at a dinner table which is not your business.
Now if it upsets your child to share them, then I would say kindly go and get them back.
I don't bring toys to communal places without expecting to share them. Kids feelings at that age are EASILY hurt and if their parent chooses not to discipline them on the matter, they will not understand why you are and they will take it personally.