One can only assume about the existence of any "unnoticed thoughts or ideas" regarding the Victor's creature in Mary Shelley's novel Frankenstein.
For the most part, the creature is very upfront with Victor, and the readers, about his thoughts and actions. Outside of that, any speculations made can only be assumptive and are only justifiable through textual evidence. Therefore, the reality of the creature's contaminated heart exists for each individual reader and their own personal understanding of the creature's story.
One such assumption, which could be made, is that the monster's contaminated heart is a result of his own innate evil. While some critics have argued the fact that the creature was a result of the society to which he was brought into, others have argued that the creature was simply a product of his own nature. By stating this, one would be assuming that the creature's heart was contaminated well before it was placed into his body by Victor.
Therefore, the creature's heart was a product of the contamination placed there by the person to whom it belonged to prior to Victor using it to reanimate life. Think of it this way, if a damaged heart is placed into a healthy person, eventually the damaged heart will affect the health of the person.
Basically, the main point being made here (or the "unnoticed thought or idea") is the fact that the creature's contaminated heart was a product of the person from who it came. The fact that the heart belongs to the creature now holds no relevancy.