If an agent takes just a couple of your brochures, you could feel optimistic. Maybe that person will have a ready-qualified potential buyer to offer you, and they equipping themselves with relevent information.
Taking the whole stack is suspicious, I'd say. More like trying to interfere with your process.
Such a person is unlikely (one hopes!) to be an actual Realtor. There is a strict Code of Ethics that Realtors deliberately sign on to, with considerable seriousness. "Realtor" is not a synonym for "Real Estate Agent".
Since you are clearly observing this activity (otherwise you would be expecting that the brochures were being picked up by members of the public) I think it would be a good idea to intercept and introduce yourself. You could ask for the card of each person, in friendly fashion.
EITHER you would develop a positive relationship where, even though he/she is not listing your property, they might nonetheless think of your property when they have clients needing what you have, in your chosen price-range.
OR you would recognize the individual next time you see your whole supply being removed in one go.
In that event, you should call their Owner-Broker or office manager, as per the card.
The office owner is likely to have, themselves, subscribed to Realtor status and would thus abhor unethical behavior. (I still keep hoping that there's an innocent explanation!!)
If you got no satisfaction, you could invoke the Code of Ethics and complain to the local Board of Realtors.
Having the person in question on your video-tape already, means you could jump straight to the last step, if you did not want to speak to that person directly. Interfering with your process, intentionally, seems like bad behavior!