'A spotter looks for or observes a particular thing as a hobby or a job, for instance, a person who observes weather patterns for an organization.'
In the area of health and sanitation, perhaps a spotter is an observer or even an inspector.

"PPE spotters are staff who provide coaching on donning and doffing (putting on and taking off PPE). They are particularly important in the setting of high numbers or outbreaks of COVID-19 cases to ensure the appropriate use of PPE and to prevent HCW infection."

"Personal Protective Equipment (PPE) Spotters play an important role in protecting healthcare workers from the risk of transmission of coronavirus (COVID-19). Their role is to model and elicit best practice approaches to PPE through providing guidance, support and leadership. The Department of Health and Human Services has issued guidance which outlines the expectations and responsibilities of the PPE Spotter role."

As you stated previously, this person works in a warehouse.

These are job descriptions for warehouse spotters -

"The Warehouse Spotter is responsible for spotting trailers, processing inbound and outbound orders, performing inventory management practices, verifying and recording material handling transactions, while following all safety and quality procedures and proficiently operating all material handling equipment. The Warehouse Spotter must also possess a great deal of flexibility in their ability to multi-task and maintain a positive, team attitude."
https://www.indeed.com/viewjob?jk=0c82299fe7f4b37f&tk=1fg8u7...

"Safety: Diligently protects the safety of themselves and others; follows all safety regulations and checklists
Quality: Conducts visual inspections of docks and trailers.
5S – Housekeeping - Maintain clean, safe, and efficient spotter/work area – dispose of trash in correct disposal methods. Yard and dock areas must remain clean and free of debris.
Manage yard inventory of trailers needed/spotted in doors for material replenishment to the production lines and for finished goods loading.
Safely and efficiently move trailers between pads, doors, and buildings on a timely basis.
Daily yard inventory taken and reporting of any trailer damage/repairs need to be reported to the supervisor/team lead.
Fill out spotter log daily
Follow all shipping safety procedures including trailer inspections and trailer lock out procedures (jacks, chocks, dok lok)
Work with warehouse personnel to plan changeovers and line pullers for finished goods trailers..."
https://www.ziprecruiter.com/c/KIK-Custom-Products-Inc./Job/...

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Note added at 7 hrs (2021-09-23 15:36:21 GMT)
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Outside in the warehouse yard, a spotter is someone who drives a spotter vehicle, moving trailers to and from the spot where they are dropped off/collected and where they are unloaded/loaded -

"Spotting
Placing a container in a position from which it can be loaded or unloaded."
https://www.shippingsolutions.com/blog/a-glossary-of-interna...
https://www.truckingpodcast.com/lessons-learned-driving-a-ya...

However, I have since found out that there are also spotters inside the warehouse that work with forklift operators. They direct the forklift operator and watch out for (spot) potential hazards that the forklift operator can't see and communicate this to the operator.
https://careertrend.com/info-10038968-forklift-spotter-train...