Final answer:
Heather should report the unknown spill to her supervisor or the appropriate personnel to maintain laboratory safety.
Step-by-step explanation:
When Heather arrives at the lab and notices a puddle of a yellowish liquid on the floor near the door, with no clear indication of its origin, she should report the spill to her supervisor or the appropriate personnel. This is in accordance with standard laboratory safety protocols, which prioritize the reporting of any accidents or spills immediately to ensure that proper procedures are followed.
By doing so, she helps maintain a safe environment for everyone present in the lab.