Massei remarks that the stained bathmat on the clean floor is itself an obvious evidence of cleanup.
The fact is Amanda's blood was in the bathroom, on the faucet and on a plastic box. This, itself, cannot be dated. But contextually, it can be: Amanda dated it by saying it was not there the day before, and she did not provide possible explanations for why it could have fallen there in the morning. It was in a very visible place, where it would have been cleaned by Meredith if was there before, and we don't have memories by Amanda about blood losses (blood gets on finger, body etc) even the day before. We have all elements to define "unlikely" that this blood stain is unrelated to the context, since there is no element that would make it likely outside the context. It is not normal that one doesn't know, did not realize, if he/she had a blood loss the day before, it is not likely to have no clue about it; and it is not normal that one does not realize if is bleeding at the moment as amande recalls t have done in her e-mail: a normal person checks immediately on his/her body if thinks a blood stain comes from his/her ear face or or body; that would be the first thing, not calculating the size of the stain to conclude it's not yours because too big.