knife

Will an object be impure if there are no signs of impurity visible?

Q:

1. If someone gets scratched by a persons nail accidently and blood begins to flow or a person gets cut with a knife and blood begins to flow but no blood can apparently be seen by the naked eye on the person (who scratched you) nail nor can blood be apparently seen by the naked eye on the knife, does the person who scratched you have to wash his nail or does the knife become napaak and have to be washed even though you can apparently see no blood with your naked eye?

2. In a similar scenario, if you are not sure when or which object cut you and caused blood to flow and no blood can apparently be seen on anything or a glass shatters and a piece cuts you but you are not sure which tiny piece that is now scattered on the floor cut you and no blood can apparently be seen on any of the pieces, do you regard everything in the vicinity as napaak even though no blood can apparently be seen by you on anything?