I always check rooms for bed bugs when we stay away. I found these two marks on the mattress protector at the end of one of the beds, as well as a washed off small (sort of bug-sized) bloodstain nearby. Mattresses, sheets etc are otherwise pristine (so is the whole house!). I even unzipped the mattress and didn't find further marks.
Would you worry? I just don't know how else to explain those stains.